Offering ____________is the most powerful strategy adopted by persistent grand-childless parents.

A．Put the homework on Don’s desk.
B．Leave the master key for Don.
C．Give Don’s students the next assignment.
D．Call Don at the end of the afternoon.

A．Wait for about three minutes.
B．Try dialing the number again.
C．Call again some time later.
D．Make an appointment with Dr. Chen.