What is a sign that indicates rapid decompression has occurred?

A door blow out serves as a significant indicator of rapid decompression because it typically results from a sudden and severe difference in pressure between the inside of the aircraft cabin and the external environment. During rapid decompression, this pressure imbalance can lead to structural failures or the rapid expulsion of objects, including cabin doors. In the event of rapid decompression, the force exerted by the cabin atmosphere can exceed the mechanical integrity of a door, leading to it being blown outwards.

Observing a cabin pressure equalize in over 10 seconds indicates a gradual process rather than a rapid event, negating the premise of rapid decompression. Cracked windows could suggest a problem but are not definitive signs of decompression. Additionally, a gradual loss of cabin pressure reflects a slow leak rather than an instantaneous change that characterizes rapid decompression. Thus, the occurrence of a door blow out is a clear and alarming sign of a rapid decompression event, making it the correct choice in this context.