Experiments of this kind __________ in both the U. S. and Europe well before the Second World War started.
Is ____ dangerous to swim in the river alone?
--___do you clean the reading room? ---Every two or three days.
Jane goes to Paris to spend her holiday ____.
–He is a doctor and his brother is a doctor, too. –You mean both he and his brother________ doctors, right?
–Mimi knows so much about the earth! –Yes, you know she is only _______ eight-year-old girl.