---My new job is not as wonderful as I expected.

---Well, as the proverbs goes, “_________”

A．A year’s plan starts with spring.

B．A good beginning is half done

C．A light heart lives long.

D．The grass is always greener on the other side.

在下图所示的齿轮—凸轮轴系中，键2称为（）

A．零件

B．机构

C．构件

D．部件