The most important thing_______we should pay attention to is the thing_______I have said.

A．which that

B． that which

C． which which

D．that that

在下列各种情况下，埋在地下的铸铁输气管道被腐蚀速度最慢的是（　　）

A．在潮湿、疏松、透气的土壤中

B．在呈酸性的潮湿土壤中

C．在干燥、致密、不透气的土壤中

D．在含沙粒较多、潮湿透气的土壤中