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單項(xiàng)選擇題

Large pipes or down-comers are fitted between the steam drum and （） to ensure good natural circulation of water.

A.fire-tubes
B.furnaces
C.water drums
D.water wall tubes

單項(xiàng)選擇題

With reference to a rotary vane steering gear system, which one of the following statements is true?（）

A.The three moving vanes and the three fixed vanes form three pressure chambers
B.The three moving vanes and the three fixed vanes form six pressure chambers
C.The three moving vanes and the three fixed vanes form nine pressure chambers
D.The three moving vanes and the three fixed vanes form twelve pressure chambers