槽轮机构的主动件是（）。
A.槽轮
B.曲柄
C.圆销
D.曲柄、圆销
参考答案：B
Mike has much difficulty ________ Chinese so he often asks me ________ help.
A．to understand; with
B．understanding;of
C．to understand;for
D．understanding; for
主要造岩矿物是组成（）的主要矿物.
A、地层.
B、岩石.
