— Shall we go out this weekend?

— Oh, with so much work ____, I don't know if I'll have time to go out. [ ]

A. to do

B. doing

C. done

D. do

工作条件不变的离心泵如流量、扬程超过需要，为节省功率，最佳的工况调节方法是（）。

A．关小排出阀

B．开大回流阀

C．换变速电机

D．将叶轮外径车削减小