求助test 4－18

18.   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?

(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。。我不太明白，为什么B不对？