Experiments of this kind __________ in both the U. S. and Europe well before the Second World War.
— Why weren't you at the meeting? — I ________ for a long distance call from my sister in America.
We have done things we ought not to have done and ________ undone things we ought to have done.
The two ladies are ________ the same age, but there seems to be ten years ________ them.
The train ________ arrive at 11∶30, but was an hour late.
While building a tunnel through the mountain, ________ .