The researchers have conducted an experiment to prove that people. will act in emergencies when_____.

A．When a person tries to help others, he must be clear that there is a real emergency.
B．When a person tries to help others, he should know whether hey are worth his help.
C．A person must take the full responsibility for the safety of those in emergencies if he wants to help.
D．A person with a heart attack needs the most.

A．to explain why people fail to act in emergencies
B．to explain when people will act in emergencies
C．to explain what people will do in emergencies
D．to explain how people feel in emergencies