单项选择题
Ammonia when used as a refrigerant is valuable because of its high efficiency, it is however （）.
A.toxic B.flammable C.explosive D.all of the above
单项选择题 Which of the following is operated from the main engine room console on an automated vessel?（）
单项选择题 An un-loader is installed on an air compressor to （）.
单项选择题 After installing a new hydraulic pump in a system, what special attention should be given to the hydraulic system?（）
