Several weeks had gone by______ I realized the painting was missing.
The old man stood there with his eyes ______ on the picture.
Scientists are convinced ______ the positive effect of laughter ______ physical and mental health.
---I’m going out tonight.---Well, I’d rather you ______.
Bitten twice, the postman refused to ______ our letters unless we changed our dog.
______ to go abroad alone is what worries me most.