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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