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.
Some birds fly_______________in winter and some stay in the north.
The book,_______________as a surprise for his sister was lost in the mail.
The country life he was used to ______greatly since 1992.
Nancy enjoyed herself so much _________she visited her friends in Sydney last year.
Great changes ________ in that school. It is no longer _______ it was 20 years ago.