The operation could prolong his life by two or three years.

　　這次手術(shù)可使他多活兩三年。

　　Many children can't function effectively in large classes.

　　許多孩子在大班上課時學習效果不好。

　　The results can be divided into three main categories.

　　結(jié)果可分為三大類。

　　I could come next week, or the week after.

　　我可能下周來，或者再下一周。

　　How long can a human being go without sleep?

　　人不睡覺能活多久？