—May I borrow your computer?—Sorry, it is ________ at the moment.
Mrs. White showed her students some old maps from the library.
Suddenly the idea came to me I might turn to Mr. Smith for help.
It is still unknown_______ caused the accident last Saturday.
____ wants to stay in a hotel has to pay their own.
The experiment shows that proper amounts of exercise ,if _______regularly, can improve our health.