Why don’t all ice crystals in clouds immediately f

单项选择题

Questions 11 to 13 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.

Why don’t all ice crystals in clouds immediately fall to Earth

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.