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單項選擇題

（） are mounted in pairs to protect the boiler against overpressure.

A.Safety valves
B.Water tubes
C.Steam stop valves
D.Blow down valves

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單項選擇題

In a （） boiler the hot gases from the furnace pass through the tubes while the water is on the outside.

A.water-tube
B.fire-tube
C.exhaust gas
D.None of the above is true

單項選擇題

In a （） boiler the water flows through the inside of the tubes while the hot gases pass around the outside.

A.water-tube
B.fire-tube
C.exhaust gas
D.none of the above