____ for a long time, but he managed to catch up with his classmates at last.
I can’t find my English book. I must ___ it at home this morning.
--Who did you ___ in the street last Sunday?---Lin Tao, a good friend of mine.
Something ___ to save our earth. Do you think so?
I’ m sorry, I mistook you ___ your sister ___ mistake.
The scientists are trying to ___ natural energy to decrease air pollution.