21. From the passage we know that the most flexible(灵活的) way to travel is _________.
A. by car B. by train C. by plane D. by boat
22. The underlined word “They” in the passage refers to_________.
A. modern trains in the country B. the travelers on modern trains
C. comfortable seats and dining cars D. the slower ways to travel
23. The best title for this passage
A. How to travel B. Ways to travel C. Modern life D. Holiday planning
