Questions 18 to 20 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.

Mental decay due to aging is

Why don’t all ice crystals in clouds immediately fall t...

单项选择题Why don’t all ice crystals in clouds immediately fall to Earth

Before the Bell Company became a publicly held corporat...

单项选择题Before the Bell Company became a publicly held corporation, how were the calls passed to their destination

What was the cause of the explosion according to people...

单项选择题What was the cause of the explosion according to people there