The teacher was absorbed in writing something, only _____ now and then to refer to a book or another.

A．to stop

B．stopped

C．stopping

D． having stopped

已知某城市最高时总用水量为280L/s，其中工业集中用水量为10L/s，在节点3处取出各管段长度和节点编号如下图所示，泵站至节点4两侧无用户。则该管网的比流量为（）L/（s·m）。

A．0.035

B．0.5

C．0.05

D．0.005