Why was the Great Wall built?-To ________ against enemies.[]

If youtalk about it with me,please wait till I finish my work at hand.

_______too late last night, I could not have fallen asleep in class

thor of the article believes that it isthe test system, ______ teachers, that is responsible for the heavy burden of themiddle school students nowadays.

When the man came in, gun in hand, we all stood there, ________．