单项选择题 If a centrifugal pump is driven by a constant speed electric motor and produces a discharge pressure less than designed, the cause is probably（）.

单项选择题 Which of the following statements is correct relative to distillation plant operation?（）

单项选择题 The amount of moisture in a given sample of air, when compared with the amount of moisture the air could hold if totally saturated at the existing temperature of the sample, is called （）.