单项选择题
When the heat load begins to increase, the needle valve of an expansion valve will（）
A.move in an opening direction
B.move in a closing direction
C.either open or close
D.not move at all
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A.help preheat the air for the furnaces
B.help preheat the feedwater
C.protect economizer from excessive heat
D.prevent excessive furnace heat losses -
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