问题描述：
He looked at the paper for three hours.He didn't write a world.(两句合一句）
He looked at the paper for three hours ---a word---.
He is very clever.He can work out the problem.
It is---of---to work out the problem.
He looked at the paper for three hours ---a word---.
He is very clever.He can work out the problem.
It is---of---to work out the problem.
问题解答：
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