Here are the corrections for each sentence:

31. **A** - "know" should be "known" (How long have you known your teacher?)
32. **C** - "times" should be "time" (I have had a wonderful time in Ho Chi Minh City.)
33. **B** - "is" should be "was" (What would happen if the temperature were warmer?)
34. **A** - "where" does not fit and should be removed (Da Lat, located on the Highlands, is famous for its mild climate.)
35. **D** - "are" should be "is" (One of the worst diseases mankind has ever faced is cancer.)

So the marked letters for corrections are:

31. A
32. C
33. B
34. A
35. D