单项选择题
Propeller running at （） are generally efficient.
A.high speeds B.medium speed C.low speeds D.the same speed with the engine
单项选择题 Which is not the maintenance work for shaft bearings in transmission system?（）
单项选择题 Water lubricated tailshaft bearings wear,（）
单项选择题 When sailing in cold zones, the（） should be drained of water.
