This problem is said _______ three times, but he still can’t understand it.

A．to have been explaining

B．to have explained

C．having been explained

D．to have been explained

单位掘进循环开挖土量，一般按( )计算。

A．V＝100Q/(Gs·W+100)
B．Q＝πD²St/4
C．V₃＝[(G₂-1)Q₂-(G₂-1)Q₂]/(G₂-1)
D．Q＝λπ(D²-d²)L/4