根据对话内容，在每小题的空白处写出一个完整的句子，使对话意思完整。

John: Oh, Tom. Please don't sit on this chair!

Tom: 1.                                     ?

John: Look! One of its legs is broken. It's dangerous to sit on it.

Tom: 2.                                     . Who's broken it?

John: I don't know. I found it this morning when I was cleaning the floor.

Tom: Let's tell our teacher about it and then someone can repair it.

John: No. 3.                                     .

Tom: Yourself? Have you learnt to repair a chair?

John: Yes. My father is a worker. He is good at repairing things. 4.                                   . 

Tom: Oh, your father taught you. 5.                                   ?

John: Certainly. Let's begin.

透明宝石在正交偏光镜下转360°均消光，在晶体光学中称为（），该宝石的光性可能是（）或（）。