齿轮的三要素是（）。
A、齿数、模数、齿宽
B、齿数、齿高、压力角
C、齿数、模数、压力角
D、齿数、齿宽、压力角
参考答案：C
As we know, paper is made wood, wood is made paper.[ ]
A. of; from
B. from; into
C.into; by
D. in; from
The volunteers for the Shanghai World Expo are busy from dawn ______ dust but they feel very proud of it.
A．at
B．on
C．till
D．in
