31. In the past few years, research has suggested that Vitamin D may increase the level of serotonin（血清素）, one of the key neurotransmitters influencing our mood, and ________it helps to relieve mood disorders.
| A．that | B．where | C．what | D．Which |
Listening to loud music at rock concertscaused hearing loss in some teenagers.
| A． | is | B． | are | C． | has | D． | have |
I walked slowly through the market, where peopleall kinds of fruits and vegetables.
I studied the prices carefully and bought what I needed.
| A． | sell | B． | were selling | C． | had sold | D． | have sold |
Tim is in good shape physicallyhe doesn't get much exercise.
| A． | if | B． | even though | C． | unless | D． | as long as |
Cindy shut the door heavily and burst into tears. No one in the office knew__________she was so angry.
| A．where | B．whether | C．that | D．why |
I'm tired out. Iall afternoon and I don't seem to have finished anything.
| A． | shopped | B． | have shopped |
| C． | had shopped | D． | have been shopping |