单项选择题

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.

What does the passage tell us about American rivers

A.One sixth of them are seriously polluted.
B.One third of them are seriously polluted.
C.Half of them are seriously polluted.
D.Most of them are seriously polluted.