单项选择题
The successful operation of any engine or machine and （） depend on effective lubrication.
A.their no trouble period B.their freely trouble period C.its free trouble life D.its trouble-free life
单项选择题 The starting of a marine diesel engine is generally carried out with the assistance of （）.
单项选择题 The starting air system usually has （） to prevent starting if anything is not in order.
