Everyone knows that _____ sun is in the center of the solar system（太阳系）

George, how can you prove the earth is round?—I can’t, sir. ______, I never said it was.

The boy is ____ young to join the army (参军). [ ]

I spent the whole day repairing the car. The work was______ easy.

He often talks ______ but does ______. People like him.