It's OK to________an electrical appliance on so long as you are using it.
— You haven’t lost the ticket, have you?— _________. I know it’s not easy to get another one at the moment.
Eating too much fat can _______ heart disease and cause high blood pressure.
--- Tom, why didn’t you come to the meeting yesterday? --- I _____, but my car broke down.
There is no doubt ______ man will conquer nature some day.
______ different life today is from ____ it was 20 years ago.