Questions 19 to 21 are based on the passage you have just heard.

Directions:In this section, you will hear three passages. At the end of each passage, you will hear three or four questions. Both the passage and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A, B, C and D). Then mark the corresponding letter on Answer Sheet 1 with a single line through the centre.
  • 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.
  • A Report their illnesses.
  • B Offer blood samples.
  • C Act as research assistants.
  • D Help to interview patients.
  • 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.