請依下文回答第 51 題至第 55 題：Backdrafts are of great danger. They often   51    even highly experienced firefighters. A backdraft canoccur when a compartment fire has little or no   52   . Due to this, little or no oxygen can flow into thecompartment. Then, because fires   53   oxygen, the oxygen concentration decreases. When theconcentration becomes too   54   to support combustion, this may cause a phenomenon in which whenmaterial is heated enough, it begins to break down into smaller compounds, including hydrogen. However,the hydrogen and smoke remain at a hot   55   enough to auto-ignite. If oxygen is then re-introducedto the compartment, e.g. by opening a door or window to a closed room, while the gasses are still hotenough to auto-ignite, combustion will often restart abruptly.
【題組】51
(A) mourn
(B) revere
(C) delight
(D) surprise