在硫酸铁溶液中，加入a g铜，完全溶解后，又加入b g铁，充分反应后得到c g残余固体，且b<c，则下列判断正确的是                           (　　)

A．最后得到的溶液中含有Fe^3＋

B．残余固体可能为铁、铜混合物

C．最后得到的溶液中只含Cu^2＋

D．残余固体一定全部是铜

When using a high-velocity fog stream in a passageway，the possibility of a blow back must be guarded against. Blow back is most likely to occur when（）.

A.pressure builds up in the nozzle which causes a surge of water

B.the only opening in a passageway is the one from which the nozzle is being advanced

C.pressure in the fire hose drops below 100 psi

D.a bulkhead collapses due to heat and pressure