人教版小学五年级下英语中有这样一段打电话句子.Mike:Hello.Larry:Hello.T

　　人教版小学五年级下英语中有这样一段打电话句子.

　　Mike:Hello.

　　Larry:Hello.ThisisUncleLarry.How'severybodydoing?

　　Mike:Justfine.Grandpaiswritingaletter.Brotherisdoinghomework,Momiscookingdinnerinthekitchen.Iamcleaningtheroom.请问How'severybodydoing?为什么用is