The operation could prolong his life by two or three years.
这次手术可使他多活两三年。
Many children can't function effectively in large classes.
许多孩子在大班上课时学习效果不好。
The results can be divided into three main categories.
结果可分为三大类。
I could come next week, or the week after.
我可能下周来，或者再下一周。
How long can a human being go without sleep?
人不睡觉能活多久？