下列各式中①$\sqrt{a}$；②$\sqrt{b+1}$；③$\sqrt{a^2}$；④$\sqrt{a^2+3}$；⑤$\sqrt{x^2-1}$；⑥$\sqrt{x^2+2x+1}$，一定是二次根式的有（　　）个．

从方框中选择最佳选项补全对话，其中有两项多余。

A: What are you doing?
B: I’m reading a letter from my pen pal.
A: ____
B: A girl.
A: Where is she from?
B: ____ 
A: What does she look like?
B:____ 
A: Is she tall?
B: ____ . She is of medium height. 
A: Look at her photo.
B: Oh. She’s good-looking and a little thin. Can she speak Chinese?      
A: ____

正确书写词语。