By the time they reached the bus stop, the bus__________ for ten minutes.
A:had left
B:have left
C:had been away
D:have been away
用“分子的观点”解释下列现象,不正确的是( )
根据图中所给的折射光线,补充相应的入射光线.
下列减弱噪声的措施中,属于在传播过程中减弱噪声的是()