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

A．He rushed into a shop the last minute before it was locked.
B．He was locked in a store while the staff hurried home.
C．He received a lot of anonymous Christmas presents.
D．He beat the owner dead when breaking in the store.

A．The quicker it vibrates, the greater the frequency of sound it produces.
B．The slower it vibrates, the higher the frequency of sound it produces.
C．The quicker it vibrates, the lower the frequency of sound it produces.
D．The frequency of sound it produces is not in proportion to the speed it vibrates.