A) If you don't study, you won't do well on your exams.
But it is much stronger in tone than A). The understanding is that the person taking the test has not been studying. It's similar to,
Well, because you haven't been studying you probably won't pass the test.
Is it because two negatives make an affirmative
You may have gotten this idea from the prescription against double negatives, Zahari, but I can assure you that the "proof" for that "rule" is completely specious.
"I don't got no money"
does not mean, to any speaker of English that the person who said that has money.