单项选择题

Questions 17 to 20.are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions.
Now, listen to the passage.

How were the police able to prove that Joe had robbed the bank

A.The raid had been photographed by hidden cameras.
B.Some watchmen had seen the raid.
C.The bank teller proved that Joe was the robber.
D.Some monitors had been installed nearby.