The students _____ are hard-working and ready to help others should be praised.
We observed some young men __________into the building at 11:00 last night.
Hangzhou is __________beautiful city,and it is worth__________.
The boy is old enough to __________the difference ________right and wrong.
He fell in love__________Mary and asked her to marry__________him.
We laugh at jokes, but seldom __________about how they work.