6.A.Beijing. B.Shijiazhuang. C.Xi'an. 17.A.By plane. B.By train. C.By bus. 18.A.14. B.13. C.12. 19.A.China. B.France. C.Canada. 20.A.At 6:10. B.At 6:30. C.At 6:40. 21.A.Two. B.Four. C.Six. 22.A.Mike and Mary. B.Mike and his mother. C.Mary and her father. 23.A.Mike's uncle. B.Mike's father. C.Mary. 24.A.Play games. B.Send and receive e-mails. C.Talk with his friends. 25.A.Because he thinks it's very exciting. B.Because he thinks it's very useful. C.Because he thinks it's very important.