@jac376 said:
Flaw: The correlation of Ra, C, and (C --> Rh) does not mean there is a causation from C to Ra. The answer which strengthens this argument will show that (Rh --> Ra) (C causes Ra because it causes Rh which causes Ra) or directly ...
Interesting. You're positive the NA isn't actually necessary? I'm curious about these examples. I didn't look closely at it, but PT 71.S1.Q11 seems to be about pollution. Is that the right reference? PT means PrepTest, right?