Which of the Following Cases Is the BEST Example of an Instance

Which of the following cases is the BEST example of an instance of social contagion?

A) A teacher in a school smells 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.