In those days bikes were not ______ used because few people could afford (负担起) to buy one.
After a heavy rain , There is ___________ water on the playground .
If he ________harder , he will catch up with us soon .
Beijing is one of the _________________ in the world today .
I don’t know if he __________ tomorrow . If he _______ here , I will call you .
Can you __________________ other ways to solve this problem?