单项选择题 A process of comparing the actual performance of a machine with the intended performance, and the adjusting the machine to reduce and eventually eliminate the difference between the actual and intended performance is called（）

单项选择题 The gauge pressure inside a container is -0~5kg/cm2, then what is the absolute pressure exerted to the inside surface of thin container?（）

单项选择题 The crane jib is raised and lowered by means of （）