蒙对的，大家给些非蒙的思路，呵呵~

寂寞的自由

16. ???！！！The average age of chief executive officers (CEO's) in a large sample of companies is 57. The average age of CEO's in those same companies 20 years ago was approximately eight years younger. On the basis of those data, it can be concluded that CEO's in general tend to be older now.Which of the following casts the most doubt on the conclusion drawn above?   C
(A) The dates when the CEO's assumed their current positions have not been specified.
(B) No information is given concerning the average number of years that CEO's remain in office.
(C) The information is based only on companies that have been operating for at least 20 years.
(D) Only approximate information is given concerning the average age of the CEO's 20 years ago.
(E) Information concerning the exact number of companies in the sample has not been given.
答案是C，这个题，我现在还是没有自信的思路来得到正确答案，当时是瞎蒙的！！！大家砸完鸡蛋，给自由些思路以及如何面对这样的削弱题。

qww

一个调查或研究是否具有代表性关键要看所选取的样本数量和代表性，第一样本数量一定要够，第二要随机选取（不能有特定性）。
C说样本的选取有特定性，故不能代表整个集合的特点。消弱。可结合数学的概率问题考虑，就清楚了。
其它是无关选项。out of scope。

寂寞的自由

明白了，谢谢哦，让我加深理解体会逻辑题的大原则：）