16Questions 16 to 18 are based on the passage you have just heard
A)They help farmers keep diseases in check.
B)Many species remain unknown to scientists.
C)Only a few species cause trouble to humans.
D)They live in incredibly well-organized colonies.
17
A)They are larger than many other species.
B)They can cause damage to people's homes.
C)They can survive a long time without water.
D)They like to form colonies in electrical units.
18
A) Deny them access to any food.
B) Keep doors and windows shut.
C)Destroy their colonies close by.
D)Refrain from eating sugary food.
19Questions 19 to 21 are based on the passage you have just heard.
A)The function of the human immune system.
B)The cause of various auto-immune diseases.
C)The viruses that may infect the human immune system.
D)The change in people's immune system as they get older.
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