What do you think of the book?— Oh, excellent.It’s worth _____ a second time. [ ]

I saw him leave the office an hour ago . He ____ at this time .

There are fifty students in our class, _____ of us _____ football.

Great changes ______ in China in the past 20 years.

237. Our teacher will _____ our class a picnic at the riverside tomorrow if the weather permits.