to me, in A), the it comes right after the word cancer, so to me it seems like a pronoun ref error or at least its not clear whether the first it in the underlined portion is refering to breast cancer or cancer.

im having trouble with this. how do we know where there is truly a pro noun reference error or not.
Why, in this sentence does the first it not cause ambiguity and is correct?
thanks.

The reason here is that the subject of the clause before the comma is (breast) cancer.
Because of this, it is clear that the "it" just after the comma refers to (breast) cancer.

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