I was going to ask if anyone could help me out with answer choice A and B, but I think I figured it out.

(A) You don't know that cell death OFTEN does not occur in test animals. The stimulus does not support this answer choice.

(B) "Until now studies ... have always involved administering to test animals doses of additives larger than the largest possible human exposure levels and massive enough to kill large numbers of cells in the animals, and then observing the ... rate" definitely supports our answer choice (B), which says that results encouraged overestimation!