单项选择题
Lubricating oil should be changed on a heavy duty diesel engine when（）.
A.it gets dark in color B.a sample rubbed between fingers feels thin C.it has been in use for a specified interval D.it no longer supports combustion
单项选择题 If your propeller is racing in rough weather，you should（）.
单项选择题 If you are unable to stop a diesel engine by any other means，you should（）.
单项选择题 If ignited，which material would be a class B fire?（）
