离心式压缩机与活塞式压缩机相比具有()特点。
A.振动大
B.运转平稳;效率高
C.效率
D.功率大
参考答案:B
y=kx+b是一次函数,则k为( )
A.一切实数
B.正实数
C.负实数
D.非零实数
Mrs.Black had such a wonderful time she visited her friend in London last year
A.when
B.that
C.which
D.as