Questions 11 to 13 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage. Why should one reply to an invitation promptly

A．It is a custom observed by every people in the world.
B．The hostess may want to have a certain number of guests at her table.
C．Otherwise one would not be invited again.
D．The host may cancel the meeting if one fails to reply.