He had no choice but to go on because it is their agreement that no one ______ quit the plan half way.

 Everyone ______ be polite to the old people.

—Must I finish my homework at school?— No, you_____. you can do it at home

Please remind your grandpa to take medicine on time, for a man of his age ____be very forgetful．

--- The fog is so thick that the express way ____ have been closed. --- You are right. I ____ have thought of that. Now we have to take another road.