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.
20
A)Report their illnesses.
B)Offer blood samples.
C)Act as research assistants.
D)Help to interview patients.
21
A)Strengthening people's immunity to infection.
B)Better understanding patients' immune system.
C)Helping improve old people's health conditions.
D)Further reducing old patients' medical expenses.
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