Questions 19 to 21 are based on the conversation you have just heard.

A．Take one dictionary out of the room.
B．Borrow a dictionary somewhere else.
C．Borrow a dictionary from the library.
D．Buy a dictionary of his own.

A．Those with frequencies lower than 20 hertz.
B．Those with frequencies about 21,000 hertz.
C．Those with frequencies lower than 20,000 hertz.
D．Those with frequencies higher than 20,000 hertz.