根据句意及句首字母提示完成句子A hundred years is a c----------- .You

根据句意及句首字母提示完成句子
A hundred years is a c----------- .
You must k-------- at the door before coming in .
The temperature will fall b-------- zero .
Jack is coming up to the blackboard.He`s walking t-------- the teacher`s desk .
We d-------- them into two groups according to their ages .