单项选择题
A.continuously B.regularly C.constantly D.period
It is necessary for us （） after having served for a cer...
单项选择题It is necessary for us （） after having served for a certain period.
A.to change the lube oil B.have the lube oil purified C.have the lube oil burnt D.to analyze the lube oil
It is important to use only pure lead and to choose a t...
单项选择题It is important to use only pure lead and to choose a thickness of wire which is greater than the normal allowable clearance but （） this amount.
A.not less than twice B.greater at least twice than C.not greater twice than D.larger three times than
It is important that the water （） very often as this ca...
单项选择题It is important that the water （） very often as this can lead to increased deposits.
A.not be changed B.be changed C.should not be changed D.should be changed
