When you say "A is the cause of B", that implies that if we want to stop B, we have to stop A.
I say, "C is a much more significant cause of B", to which you reply, "no, it's C+A that causes B".
I say, "C+A is a subset of C and all C causes B. Also, C+A is just a subset of A. Ergo we should eliminate all of C, not all A"
Is that clear?