从各题A. B, C.D四个选项中，选出可以填入空白处的最佳答案。
21. The trains leave every 15 minutes； if you miss______, you don’t have to wait long for another.
A. it B．this C. some D. one
22. It is...
Train big cats in Las Vegas!
Big cats are lions, tigers, cheetahs, etc.
They appear in world famous circus performances!
(No experience necessary)
•an exciting star-...
Jack is a twenty-year-old young man. Two years ago, when he finished middle school, he found work in a shop. Usually he works until ten o’clock in the evening. He is very tired when he gets home. After a quick supper he goes to bed and soon falls asle...