Questions 11 to 18 are based on the conversation you have just heard

A．Big waves pose a threat to commercial navigation.
B．The river current never flows faster ten miles per hour.
C．The river reverses its flow several times a day.
D．High tides can create sudden and unexpected rapids.

A．To observe people’s attitude towards strangers.
B．To see how people get along with their friends.
C．To change people’s behaviour in social life.
D．To find out how shy people are.