—Why does the river smell terrible?

—Because large quantities of water__________．

A．have polluted

B．is being polluted

C．has been polluted

D．have been polluted

复面洞做成阶梯的作用不包括（）

A.分散牙合力

B.少切割牙体组织

C.利于充填

D.保护牙髓

E.抗力形