[求助]fei fei-test 4-61

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.
A.       文中第一句the fatal airplane crash last August已经客观存在，说明飞机旅行并非相当安全。与文中事实相反。
B.       用于识别和报告机械故障问题的更加严格的新规定已经生效，这才是the fact that reports of airplane mechanical problems have increased in frequency by 50 percent since last August的真正原因。而B选项Mechanical problems in airplanes是它因，削弱结论。
C.       正确。“Mechanical problems in airplanes have increased by 50 percent since last August”是用他因来削弱结论，加上NOT，则是排除他因的假设。
D.       诉诸于航空公司的主观意愿，与过去相比较，都跟现在的情况无关。
E.       become easier to detect无关词。题目应该是问原argument基于什么样的假设吧。这题选C是什么意思啊？该怎么理解？麻烦大虾们帮我解释下。谢