He hasn't finished his homework _____. [ ]
A. already
B. yet
C. never
D. ever
答案:B
采用“牵引辅助制动法”施行旅客列车站外调速时,应如何操纵?
施工噪声的类型中,属于机械性噪声的是( )。
A.空气压缩机发出的噪声B.木材加工机械发出的噪声C.变压器发出的噪声D.鼓风机发出的噪声