In the past few years research has suggested that Vitamin D may increase the level of ser

问题补充：

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

答案：

(答案→)A. that

B. where

C. what

D. which

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