If and only if is a biconditional. "b if and only if a" means "if b then a" AND "b only if a". B only if A here means "It is an apple only if is a fruit", in other words, "if it is a fruit, it could only be an apple." Which ain't right.

B -> A (if B, then A) (if apple, then fruit, correct)

B <-> A (B if and only if A) (if apple, then fruit, AND if fruit, then apple, incorrect).