Questions l4 to 17 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.
Before the Bell Company became a publicly held corporation, how were the calls passed to their destination

A．Children playing with fire.
B．The setting off of fireworks in a house with explosives.
C．Sparks from a fireworks display falling on the warehouse.
D．The firing of a gun.

A．They are balanced by the pressure of rain droplets.
B．The effect of gravity at high altitude is random.
C．They are kept in the sky by air current.
D．They are moving about with fixed direction.