The boy is my brother.The boy is standing at the door.合并:The boy who is standing at the door is

题目

The boy is my brother.The boy is standing at the door.合并:The boy who is standing at the door is
The boy is my brother.
The boy is standing at the door.
合并:The boy who is standing at the door is my brother.
省略：The boy standing at the door is my brother .
我的问题是：为什么合并后的句子中who和is可以省略？

答案