Which of the following statements concerning the water tube and fire tube boilers is not true?（）
- Anatural circulation of the water in the water tube boiler is better than that in the fire tube boiler
- Bfor the same output the water tube boiler will carry less mass of water compared with the fire tube boiler
- Cthe water tube boiler will cost shorter time for steam raising
- Dthe water tube boiler require lower skill in operating and maintaining