急求解释！

雅西吉

61. 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.
为什么选C 啊？非常费解……