有消声要求的通风与空调系统，室内允许噪声级为35～50dB(A)时，则主风管风速应为( )m/s。

A．4～7
B．7～8
C．6～9
D．4～8

When father was young, he（）from morning till night.

A.was made work

B.was made working

C.made to work

D.was made to work