It is rationalized when you choose which ants go into which container. The ants were chosen because of karma. To you it seems random, to the believers that's karma at work.

i'm afraid you might have a different perception of what 'karma' is. hinduism has a different perception on karma and so is jainism. the term 'karma' exists even way before the Buddha came to be. for the sake of discussion, i assume you're referring to 'karma' in buddhist context, so we can discuss coherently.

the term 'karma' in buddhist context is a 'volitional action', the intention or motive behind an action. it forms in our minds, all beings' minds, that precede any bodily or verbal actions. 'karma vipaka', the ripening or maturation of karma, is bound to happen when 'karma' is done. meaning the fruition of karma is guaranteed. the severity level of 'karma vipaka' depends on the severity of 'karma' done. it can ripen in the very present life or far future into the next life.

there are causes that drive us to have volitional actions (karma), I won't explain much of that because it is not relevant to your question but it is important to mention. to put it simply, the driving factors are the roots that manifest in 'karma' that can be categorized into unwholesome root (greed, hatred, delusion) and wholesome root (non-greed, non-hatred, non-delusion) they are in always dormant state, eager to jump out when conditions are met. buddhist monks' goal is to eradicate these unwholesome root, completely.
okay, now let us go back to your question at the very beginning: "Is karmic action over our next life guaranteed ?"

in my opinion and understanding, the result of our next life can be guaranteed only if certain conditions are met. and exactly what "conditions' as you ask further previously, that would depend on what sort of result you're expecting to achieve. with the right cause and nourished with the right conditions correspond to the expected result, the expected result would then be guaranteed.

if we refer back to your ant experiment. the CONDITION here for the early death of ants in container A is the administering of pesticide. the story doesn't end there. life cycle goes on in Samsara. the ants died may reborn into ants again or other beings in the next life, depends on the expiration & maturation of karma of the deeds of their past lives. 

actual buddhists practice the following 3 steps: dana(generosity),  sila(morality & virtue), then bhavana(mental cultivation, also known as meditation) dana -> sila -> bhavana

by putting effort and gradually training in accordance to these steps, you're conditioning the mind to lean towards pure and clean state. pure and clean state of mind is a happy mind 

once sila(morality) are like your second nature, what comes naturally is samadhi(concentration), a calm and serene mind that's clear, even blissful state of mind, which help ones to see things as they are, in other word, the reality la. that's when panna(wisdom) come to be.

sila -> samadhi -> panna.
"what 'things' ahhh???" you may ask. those 'things" are the reality of anicca(impermanent), dukkha(unsatisfactory-ness), anatta(non-self)

all phenomenas come to existence based on conditionality. even our state of mind, mental habits and activities and conditioned-based. they are ever changing, thus anicca(impermanence). we always want to hold on to pleasant feelings and always dispel unpleasant ones, and since they are impermanent, there are also dukkha(unsatisfactory). also because all phenomenas are subject to change,  this characteristic is called anatta(non-self) 
true or not ? hmm i don't know. you'll have to experience it for yourself. perhaps can be liken to this analogy: nearly impossible to describe the taste of durian to someone who hasn't tasted it themselves 

oops sorry, i only answered your 2nd question in the form of simplified steps guidance to achieve a clear mind.

to your first question on "how is a mind clear", a mind is clear when the driving factors - the underlying unwholesome roots in the mind that manifest in 'karma", are then subdued through practices. a "clearer", serene and calmer state of mind as a result. an analogy would be just like a clear pond with no ripples on the surface, and you're able to see what's underneath the pond with ease.
take for example ah, slight irritation & hatred might be present in your state of mind at this moment. your intention to reply would then be "tainted" by your present state of mind, responding with words that are not pleasant to the readers.

I asked, "How is a mind clear?
How is a mind clear enough to see as you described?"

Clearly two of you do not have a clear mind LOL

When you do not have a clear mind, how can you be objective?
So you have what the military calls, a trained response. Just say karma, trigger all these response.

Anyway, if you cannot answer my first 2 initial questions, then you are stuck in your loop. No need to reply. Thank you for reading.

yup, whether or not you subscribe to the believes of 'karma' & "rebirth', they are merely religious ideology. you will find your own version of Truth when you experience them personally. 

Clear or not, time will tell if karma will strike you or not if you ever hurt people ya. It could not happen now but in your next life.

It's hard to prove to you and you don't even know how large is the universe. And don't assume God or other Deity only handle one planet. Your new life could be carried forward from another planet.

Many people like you only make a judgement based on the earth and having the mentality of the earth is the only living planet.

That means the believers are saying the ants were chosen were the resultant (re)action of their past deeds and this karma chain cannot be broken.

My question will be, when can a new karma chain be created ?

What is stopping one to say "I killed somebody because of my past deeds" and that's my karma ?

Isn't that renders karma a useless piece of explanation ?

When you do not have a clear mind then you are living in delusion.
You hit next level when you continue to impose your ideology on others.
Next level is religious war and terror.

You are on your way there. Keep up the good work.

I am just explaining the karma point. Only you're touching about ideology, religious war, and terror. You can continue to hurt people if you think karma is untrue. No one can stop you.

Only ignorant will claim certain thing is untrue. The universe is so huge. Anything is possible unless you visited the whole universe.

Why do you say anything is possible as if you have visited the whole universe? You should not say anything at all unless you have visited the whole universe.

At least I won't become ignorant like you to claim certain things as untrue.

How do you know that when you have not visited the whole universe?

All the stars, planets and galaxies that can be seen today make up just 4 percent of the universe. The other 96 percent is made of stuff astronomers can't see, detect or even comprehend.

That's why it's silly to claim certain things to be untrue. The size of the universe is beyond imagination. How can you be sure no other human/Aliens on the other side?

See the bolded claim you made. How do you know that is true when you said yourself you haven't visited the whole universe. You should not say anything at all. Still don't understand?

I will only say based on my past experience, karma is likely to be true. I won't claim anything to be 100% true or proven to be a fact. Unlike TS already concluded it' to be untrue.

But that is not what you claimed earlier and your past experience is so tiny compared to the universe. What TS posted is to make some conclusion based on a thought experiment. Just like science which you rejected but demanded odd requirements. I find you are a blob of inconsistent mess.

Still based on your earlier logic you should not say anything at all ...

Based on a thought experiment is also very tiny compared to the universe. At least I said it's likely.

But ignorant like you like to make a conclusion without evidence to prove karma is untrue.

The thought experiment has nothing to do with the size of the universe.

The size is important unless you specifically mentioned Karma is untrue on earth.

For example, if you do your experiment to detect the existence of ghosts in a certain village that claimed to have ghosts roaming around. You did the experiment of 2 decades and finally, you concluded Ghost is untrue. This conclusion is not valid because the entire universe is too huge. Your sample size is too small to make sure a big conclusion.

This applies to almost anything including UFOs, Aliens, God, Deity or your favorite unicorn. 

We are not talking about the ghost, we are also not talking about UFO.

You have not even replied my first question, is karma guaranteed to be predictable ?

Did you even read what I wrote in my previous post ?

If you UFO or ghost thing falsifiable ?