已知两个力F1与F2的大小分别为10N和30N,则它们的合力大小不可能等于()
A.15N
B.20N
C.35N
D.50N
参考答案:A, D
There are many buses running in the street. Don't go _____ it. [ ]
A. cross
B. across
C. over
D. through
日照计安装高度以()为准。纬度允许误差是(),方位允许误差是()。