So far this year we _____ a fall in house prices by between 5 and 10 percent. [ ]

A. saw

B. see

C. had seen

D. have seen

为了保证氨冷器的蒸发空间，其液面太低会导致换热效果（）。

A.变好

B.不变

C.变差

D.无法判断