轴心受力构件用侧焊缝连接，侧焊缝有效截面上的剪应力沿焊缝长度方向的分布是（）

A.两头大中间小

B.两头小中间大

C.均匀分布

D.直线分布

The average age of chief executive officers (CEOs) in a large sample of companies is 57. The average age of CEOs in those same companies 20 years ago was approximately eight years younger. On the basis of those data, it can be concluded that CEOs 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 CEOs assumed their current positions have not been specified.

B．No information is given concerning the average number of years that CEOs 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 CEOs 20 years ago.

E．(E) Information concerning the exact number of companies in the sample has not been given.