I'm trying to do an excercise. I have to chose which one is the INCORRECT answer.

Although human beings aren't able to percieve such changes, it is thought the the sensitive nervous systems of some animals COULD / HAVE TO / MUST be affected by them.

The answer is apparently "have to" although I cannot understand why - its seems to be that each option is correct! Could refering to possibility and "must to" and "have to" being used to express certainty ?

have to is not as good as could or must, so have to should be chosen as the "incorrect" answer. Here's why.

Both could and must are modals of logic, i.e., involving logical possibility and necessity. have to is typically used as a modal of social interaction, i.e., involving personal obligations, and only less often as a modal of logic.

The situation described in the sentence ( nervous systems -- affected by certain changes ) seems to me to involve logic, not social interaction. It seems to me that it's a matter of saying that these nervous systems either are possibly affected (could be affected) or are necessarily affected (must be affected) by certain changes.

In informal language have to can replace must in its logical sense, but this passage appears to be very scientific, so that informal substitution doesn't seem like a good idea.

I hope this explanation helps. Emotion: smile

cheers fella!