15. It's already 10 o' clock. I wonder how it_________ that she was two hours late on such a short trip..
At such gatherings, people ______ are not necessarily your relatives or friends.
—We’d better go downstairs right now for lunch.—______ They serve food 24 hours a day.
Walking is a nice way to relax. ______, it can help reduce body weight if you do it regularly.
Having decided to take the trip, he ______ preparing for it.
—You mustn’t leave your car unlocked while you are away.—No, I ______. Please don’t worry too much.