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

中国古代的圆柱子上下两端直径是不相等的，而是根部略粗，顶部略细，这种作法称为（）