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.
22Questions 22 to 25 are based on the passage you have just heard
A. .A)His students had trouble getting on with each other.
B)A lot of kids stayed at school to do their homework.
C)His students were struggling to follow his lessons.
D)A group of kids were playing chess after school.
微信支付 
支付宝支付