He was often seen _________ with charity work last year.
Most people are________building a paper factory near here.They are worried the river will get polluted.
Yesterday evening I was playing the piano________the doorbell rang.A.whenB.beforeC.while
Tony________TV when his mother came in.
I saw him in the library yesterday.He________a book at that moment.
(北京)—What were you doing when we were playing basketball yesterday afternoon?—I________the classroom.