A、Serious. B、Uncommon. C、Usual. D、Light. B“uncommon”与“most unusual”同义，故选B。

Today’s first-aid topic is about handling a choking incident. In terms of the environment in which it occurs, choking is one of tire strangest and most unusual of medical emergencies.  First, it generally occurs in happy, lighthearted circumstances, when everything appears to be going right. For example, choking incidents often take place at parties, where there are lots of people laughing and having a good time. Second, choking can happen to anyone, young or old, sick or well, alone or with others, at other lime. Everybody should know what action to take in the event of a choking incident. It could happen to someone you know, or even to you, at any time.