Which of the Following Is an Example of Social Contagion

Which of the following is an example of social contagion?

A) A teacher in a school reports smelling gasoline and begins to feel nauseous;soon most of the school's population is also feeling nauseous.Later,it is discovered that there was never a gas leak.
B) A politician gives a very moving speech in which he argues that the nation is superior to all other nations.The crowd cheers very loudly.
C) A person is suffering from a severe injury on the side of the road.However,people driving by fail to stop to help even though they notice the injured person.
D) A shopper notices a sign in the electronics store,saying that supplies of a certain tablet are limited.The shopper decides to buy the tablet based on this information.