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.
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.
答