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.
This is one of the best novels _______ appeared this year.
President Obama made a speech for half an hour withouthis notes.
Look at the timetable. Hurry up! Flight 4026off at 18:20.
I still rememberto the Three Gorges and what I saw there.
Excuse me, sir.the driver wait outside?