Keep on trying. Never _________. I am sure you will succeed sooner or later.

1—What’s the matter with you You look soworried.– I don’t know ___________ the problem.

(2010·陕西高考)His first book ________ next month is based on a true story.

Don’t leave the water while you brush your teeth.

______ hearing the news of the success, he opened his mouth wide, and then had a big smile on his face.