So difficult ______ it to work out the problem that I decided to ask Tom for advice.

A．I did find

B．did I find

C．I have found

D．have I found

下列有关制冷系统停止顺序的表述中，正确的是（）。

A.冷水机组→冷冻水泵→冷冻水电动阀→冷却水泵→冷却水电动阀→冷却塔风机、电动阀

B.冷水机组→冷冻水电动阀→冻冻水泵→冷却水泵→冷却水电动阀→冷却塔风机、电动阀

C.冷水机组→冷却水泵→冷却水电动阀→冷却塔风机、电动阀→冷冻水泵→冷冻水电动阀

D.冷却塔风机、电动阀→冷水机组→冷冻水泵→冷冻水电动阀→冷却水泵→冷却水电动阀