answer: Mr. Zhang does not drink tea because he thinks tannin is harmful. But #Person1# thinks that tea can be good if it is well prepared. Mr. Zhang agrees, but he states if you let the tea stand too long it will still do you harm.
documents: #Person1#: Do you object to drinking black tea, Mr. Zhang?
  #Person2#: Yes, that is why I never drink it.
  #Person1#: Do you drink green tea, then?
  #Person2#: Very seldom, because tea has tannin which may be harmful to one's health.
  #Person1#: But if the tea is good and well prepared, it will do much good.
  #Person2#: What good is it for?
  #Person1#: Not only can it appease one's thirst, but it can also refresh one when tired.
  #Person2#: That's true, but it must not be allowed to stand long.
  #Person1#: Why is it so?
  #Person2#: Because if you drink a kind of tea which has been prepared for some time before, it will do you harm.