单项选择题
It is necessary to remove （） from the cooling water spaces to improve heat transfer.
A.lime B.frost C.scale D.scar
单项选择题 It is necessary to remove （） from the cooling spaces.
单项选择题 It is necessary to （） cool the various components of the machinery so that the engine can work properly.
单项选择题 It is necessary for us （） after having served for a certain period.
