单项选择题
A.关煤气→空废气交换→开煤气 B.关空气→煤气废气交换→开空气 C.关煤气→煤气废气交换→开空气 D.关废气→空煤气交换→开废气
A.2.0 B.2.5 C.3.0
A.40% B.50% C.60% D.70%