单项选择题
It is very necessary that each cylinder （） before the charged air is compressed.
A.is scavenged B.has been scavenged C.be scavenged D.was scavenged
单项选择题 It is very necessary that each cylinder （） before the charged air is compressed.
单项选择题 It is very important that each cylinder should be （） scavenged before the charged air is compressed.
单项选择题 It is necessary to remove （） from the cooling water spaces to improve heat transfer.
