In the past few years, research has suggested that Vitamin D may increase the level of serotonin（血清素）, one of the key nurotransmitters influencing our mood, and ________it helps to relieve mood disorders. 

In the past few years, research has suggested that Vitamin D may increase the level of serotonin（血清素）, one of the key nurotransmitters influencing our mood, and ________it helps to relieve mood disorders.

1. A.
   that
2. B.
   where
3. C.
   what
4. D.
   which

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

In the past few years, research has suggested that Vitamin D may increase the level of serotonin（血清素）, one of the key nurotransmitters influencing our mood, and ________it helps to relieve mood disorders. 

         A. that                       B. where                           C. what                       D. which

In the past few years, research has suggested that Vitamin D may increase the level of serotonin（血清素）, one of the key nurotransmitters influencing our mood, and ________it helps to relieve mood disorders.