If he continues like this, he will ____________ a stone only to have it drop on his own foot, just as the old saying goes.
When I got back home I saw a message pinned to the door, ______ “Sorry to miss you; will call later.”
----Have you seen____pen﹖I left it here this morning. -----Is it____black one﹖I think I saw it somewhere.
The news has spread all over the country ____ the spaceship succeeded in returning to the earth.
____ he works hard, I don’t mind when he finishes the experiment.
____, he does not know the answer.