All of a sudden, the thief walking behind a young and pretty lady ____ her purse, ____ into the crowd.

Mr. Duncan raised an unnecessary question __________he then failed to find an answer.

You’ve dropped _____ “s” in the word “acros”—Oh, ____ letter “s” should be doubled like this “across”.

He found very difficult the math problem．

People in the west ______it a rule to buyChristmas presents for their relatives and friends