Research shows that humans ______ from selfish to unselfish behavior when they are watched.
With the time __________, our happiness grew.
When he was 20 years old, he __________ doctor in the village.
With the popularity of smartphones, the micro blog __________ a good platform for the young to show themselves gradually.
—What worried him so much? —His hair is __________ grey.
It was in New Zealand __________ Elizabeth first met Mr. Smith.