The accident happened ______ a cold winter morning.[ ]
A. in
B. on
C. at
D. of
答案:B
A.mgtgθ
B.mgsinθ
C.μmg
D.μmg
全部信号每天应试验()次,以检查信号系统的动作是否可靠。
A.1
B.2
C.3
D.4