Please tell me the after service of the computer; I’m worried about        a breakdown sometime．

A．there being

B．it being

C．there is

D．it is

现组织流水施工，采用经验估算法确定某一专业工作队在某一施工段上的流水节拍，若该流水节拍的最长、最短和正常三种估算时间分别为：5 d、3.5d、4.5 d，则该流水节拍约为 ( )天。

A．4.42

B．4

C．4.5

D．5