单项选择题 Which of the listed chemicals can be routinely used to maintain the hygienic quality of potable water?（）

单项选择题 The ratio of the weight of moisture contained in a given volume of air, to the weight of moisture that the same sample would hold if saturated, is called the（）.

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（）.