键固定在轮毂槽中，键与轴槽为间隙配合，轴上零件可沿轴向移动是（）。
A.导向键
B.滑键
C.半圆键
D.平键
参考答案：B
In spring, a lot of young trees _______ in our city.
A. are planted
B. plant
C. has been planted
10．球墨铸铁是铸铁中力学性能最好的。
