Only after he had spent several nights working on the math problem _______ to wonder whether the problem was wrongly set.
If you __ go, at least wait until the storm is over.
It is human activities on the earth that are to _______ global warming.
I like the house with a beautiful garden in front, but I can’t afford______.
His glasses, _____he was like a blind man, were broken.
Experiments of this kind in both the U.S.and Europe well before the Second World War.