看图片，填空，完成对话。

A: Sally's birthday is coming. I want to give her a l.            . 2.            do you suggest?

B: Why don't 3.           her a teddy bear?

A: Oh, no. She's 4.            a teddy bear.

B: How 5.        ?

A: That's a good idea. I'll buy her a pinkdress.

试述少筋梁、适筋梁和超筋梁的破坏特征。在设计中如何防止少筋梁和超筋梁破坏？