-- In the countryside you can enjoy fresh air, blue sky and quiet surroundings.

--- _______ more and more retired people are escaping from the crowded city.

A．It is no wonder

B．It is no doubt

C．There is no need

D．There is no possibility

下面哪项是固定桥最重要的支持基础（）

A.牙根

B.牙槽骨

C.牙龈

D.粘膜

E.牙周膜