从汽缸容积利用角度看,提高活塞式压缩机的压力比是不利的,一般一级的压力比为()。
A.4~5
B.3~4
C.2~3
D.1~2
参考答案:B
食品包装用聚乙烯、聚苯乙烯、聚丙烯成形品的卫生标准中,计算重金属含量这一指标的金属是()
A.铅
B.锰
C.铁
D.汞
E.镍
The desks and seats can be used for a long period of time, they _____ the height of the children. [ ]
A. must be adjusted to
B. can be adjusted to
C. must adjust to
D. can adjust to