[求助]请教一题

 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.
  

给的答案是C，但是我觉得B貌似正确。求指教。