The context should give you some indication.
Is the quotation especially relevant to the argument being presented by
A, an argument that might be less fundamental to B's work and thus not
emphasised by B?
Seeking out B's original work can be useful: A may have misunderstood
B's argument. I have, on occasion, been rather surprised to find my own
work cited to support an argument which it clearly countered or to which
it was irrelevant.
--
Laura (emulate St George for email)