The domestic fresh water pump starters are controlled by pressure switches which operate when the pressure in the tank varies within predetermined limits as water is drawn offThe word “which” means （）.

In general, domestic flesh water tank kept under pressure by an external supply of （）.

A.forced circulating pump
B.compressed air
C.gravity tank
D.bead tank

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The abbreviation TDS about the potable water probably means （）.

A.total dissolved solids
B.temperature of distillation of sea water
C.total dissolved sodium
D.total delivery system

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