May I borrow your computer?—Sorry, it is ________ at the moment

The second Sunday of May is Mother’s Day and a time to be to our mothers.

Many people believe that ______ will take years to develop a car that does not pollute the air. [ ]

I can’tthank you more for the present you sent me the other day.—-____________.

Interest is as ________ to learning as the ability to understand，even more so.