单项选择题
The boiler is fired （） the demand for steam exceeds that available from exhaust gas heating.
A.whenever
B.however
C.wherever
D.whether
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A.negative displacement type
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A.well ventilated
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More than one stage of evaporation can take place by admitting the sea water into chambers with progressively （） pressure in the flash evaporator.
A.lower
B.higher
C.the lower
D.the higher
