螺杆传动中，蜗杆是主动件，其轴心线与蜗轮轴心线在空间交叉成（）角。
A.600
B.900
C.1200
D.130
参考答案：B
In spring, a lot of young trees _______ in our city.
A. are planted
B. plant
C. has been planted
一元二次方程2x2-x=0的解是______．
