Critical Reasoning

# 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.
OA: C

MyAnalysis:
conclusion: CEO's in general tend to be older now.
Reasoning: the current sample of companies have CEOs who have an average age of 57. twenty years ago, for the same sample of companies, the CEOs were approx. 8 years younger.
the Q asks us to weaken the conclusion. so why not choice B ? no information is given on the tenure of the CEOs. If the CEOs who were present 20 years ago, still continue to be in the office, then naturally the average age of CEOs will tend to rise. Since the argument does not give any information regarding the same, answer choice B should be the best criticism.

Kindly let me know your views and approach.
I seem to be getting such questions almost always wrong. Can someone suggest me as to where am I going wrong?

Thanks.