Which of the following statements concerning the water tube and fire tube boilers is not true?（）
A. natural circulation of the water in the water tube boiler is better than that in the fire tube boiler
B. for the same output the water tube boiler will carry less mass of water compared with the fire tube boiler
C. the water tube boiler will cost shorter time for steam raising
D. the water tube boiler require lower skill in operating and maintaining
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