If the average (arithmetic mean) of the 4numbers n + 2, 2n - 3, 4n + 1, and 7n + 4 is 15, what is the value of n ?
Investigators concluded that humanfailure was not responsible for the fatal airplane crash last August, and sincethat time new and more stringent rules for identifying and reporting mechanicalproblems have been in effect. That accounts for the fact that reports ofairplane mechanical problems have increased in frequency by 50 percent sincelast August.
Which one of the following is an assumptionunderlying the argument in the passage?
(A) Airplanetravel is still relatively safe, despite the increase in reported mechanicalproblems.
(B) Mechanicalproblems in airplanes have increased dramatically since last August.
(C) Mechanicalproblems in airplanes have not increased by 50 percent since last August.
(D) Airlinesare less reluctant to report mechanical problems than they previously were.
(E) Mechanicalproblems in airplanes have become easier to detect since last August.
题目翻译：如果n + 2, 2n - 3, 4n + 1,and 7n + 4这四个数的算数平均数为15,求n的值？
A. 文中第一句the fatal airplane crash lastAugust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thefact that reports of airplane mechanical problems have increased in frequencyby 50 percent since last August的真正原因。而B选项Mechanicalproblems in airplanes是它因，削弱结论。
C. 正确。“Mechanical problems in airplaneshave 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。
E. becomeeasier to detect无关词。