CR me Got Confused

Chaitanya_1986 wrote:276. Investigators concluded that human failure was not responsible for the fatal airplane crash last August, and since that time new and more stringent rules for identifying and reporting mechanical problems have been in effect. That accounts for the fact that reports of airplane mechanical problems have increased in frequency by 50 percent since last August.

Which one of the following is an assumption underlying the argument in the passage?
(A) Airplane travel is still relatively safe, despite the increase in reported mechanical problems.
(B) Mechanical problems in airplanes have increased dramatically since last August.
(C) Mechanical problems in airplanes have not increased by 50 percent since last August.
(D) Airlines are less reluctant to report mechanical problems than they previously were.
(E) Mechanical problems in airplanes have become easier to detect since last August.

I agree Op C can be the ans

conclusion: human failure was not responsible for the fatal airplane crash last August

premise 1: After plane crash in August, new and more stringent rules for identifying and reporting mechanical problems have been in effect

premise 2: After the implementation of rules, reports of airplane mechanical problems have increased in frequency by 50 percent since last August.

That means author is somewhere blaming to unreported mechanical problems for the crash. It means one of the possible assumption is, 50% reports related to mechanical failure are not reported earlier, when the rules are not that stringent and hence crash occurred.

Op C is actually one of the possible inference, and surprisingly, if we negate this, we can easily conclude that this is also the assumption.

Op B: Least negation test

Mechanical problems in airplanes have NOT NECESSARILY increased dramatically since last August.

so clearly it can not be the answer.