OPT:
(A). gas;
(B). Air;
(C). O2;
(D). CO2;
(E). Ca;
(F). O3;
(G). Fe;
(H). He;

Fact 1: Cells cannot survive without oxygen. Fact 2: And oxygen, O2, is the molecule to do it. Given the two facts above, what is something cells cannot survive without?
Answer:
(C).