An idea came to him ___ he might do the experiment in another way.
To my surprise, I hurried to the lecture hall only ______ that the lecture had been cancelled.
--- Hey, Mary, your backpack looks nice. When did you buy it. --- I ____ it for two weeks.
Why do you insist on buying a car ______ you have such good public transportation here?
The owner still doesn’t know _______ broke into the house and stole the money.
The scientists have made an exhaustive study of the virus _______ many people have been inflected.