Traveling around big cities by taxi can cost a lot of money but its usually to take the

问题补充：

Traveling around big cities by taxi can cost a lot of money, but its usually to take the underground train to most places.

A. amazing

B. expensive

C. convenient

D. exciting

答案：

A. amazing

B. expensive

(答案→)C. convenient

D. exciting

解析：amazing令人惊异的； expensive昂贵的；convenient方便的； exciting令人兴奋的。句意：乘出租车在大城市旅行会花许多钱，但是这通常能方便的乘地铁到大部分地方。结合语境可知选C。