填空题
When did people begin riding a new kind of bike?
填空题
Why was the bike unusual?
Because its wheels were of ______.
填空题
How high was its front wheel?
The front wheel was ______ feet high.
填空题
What' s the advantage of the bike?
The new bike was ______.
填空题
What is the disadvantage of the bike?
It was difficult for people to get ______.