Cycling is a very useful way, especially in cities _______ bike paths have been built.

A．which

B．that

C．where

D．what

在活动矫治器中，既可起固位作用，又可以起加力及连接作用的部件是（）

A.箭头卡环

B.塑料基托

C.唇弓

D.连续卡环

E.双曲簧