"........we ban hunting whales, they won't die out."
A- If b- Only if
Although the only grammatically correct answer is (A), the sentence "If we ban hunting whales, they won't die out" is somewhat ridiculous. I suspect that the the author of the test question intended to produce a sentence in which "only if" was the correct choice. However, that sentence needs inversion:
(b1) Only if we ban hunting whales will they not die out. (b2) Only if hunting whales is banned will they not die out.
But even those sentences aren't great, because "they" doesn't clearly refer to "whales," and there is the sense that "hunting" in "hunting whales" might be an adjectival participle (describing a type of whale) rather than a gerund (denoting the activity of hunting whales). I therefore recommend the following:
(b2) Only if we ban the hunting of whales will they not die out. (b3) Only if the hunting of whales is banned will they not die out.
I would more naturally use "go extinct" than "die out," but both are OK.
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