单项选择题
Between the steam dram and the smaller water dram below, large numbers of smaller diameter （） tubes are fitted.
A.fire
B.water wall
C.generating
D.down comer
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A.tightened up
B.sealed up
C.opened up
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A.use
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The defective place in the furnace should be （） with fire bricks.
A.paved
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D.dismantled
