[音频]Question 19.
A. The function of the human immune system.
B. The cause of various auto-immune diseases.
C. The change in people's immune system as they get older.
D. The viruses that may infect the human immune system.
Question 20
A. Offer blood samples.
B. Help to interview patients.
C. Act as research assistants.
D. Report their illnesses.
Question 21
A. Strengthening people's immunity to infection.
Better understanding patients' immune system.
C. Further reducing old patients' medical expenses.
D. Helping improve old people's health conditions.
[音频]Question 22
A group of kids were playing chess after school.
B. His students were struggling to follow his lessons.
C. A lot of kids stayed at school to do their homework.
D. His students had trouble getting on with each other.
Question 23
A. Join the school's chess team.
B. Visit a chess team in Nashville.
C. Receive training for a chess competition.
D. Participate in a national chess competition.
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