High-ranking security officials agree that ideology is no longer the main reason that United States citizens engage in espionage, as it was in the 1940's and 1950's; the chief motive now is money.

(A) that United States citizens engage in espionage, as it was
(B) for United States citizens engaging in espionage, as
(C) for United States citizens who engage in espionage as was so
(D) of United States citizens who engage in espionage, as it was
(E) United States citizens engage in espionage, as

OA is A. But does "it" refer to in this sentence.