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.
The sunlight came in____________ the windows in the roof and lit up the whole room.
The harder you work,____________.
--Do you like the material?--Yes, it ___________ very soft.
--Where is Xiao Wang?--I don't know. He__________ be in the library.
Bread and butter____________ for breakfast in many Chinese homes nowadays.