要实现棘轮转角大小的任意改变，应选用（）。
A.可变向棘轮机构
B.双动式棘轮机构
C.摩擦式棘轮机构
D.防逆转棘轮机构
参考答案：C
--- A high-tech exhibition in the National Museum.
--- Really? What about visiting it now? .
A．will be held
B．is being held
C．is to be held
D．has been held
前期管理的费用由开发建设单位（）承担。
