改错题：写出题中错误的表述，并加以改正。
在学习东欧剧变和苏联解体的有关内容时，小张同学做了笔记。由于粗心，出现了几处错误，请聪明的你从下面的笔记中帮他找出来并改正。
从1987年下半年开始，东欧各国风云突变。由于各国执政党的改变，资本主义制度也随之发生改变。东欧剧变以后，南斯拉夫一分为三。苏联在赫鲁晓夫上台后，政治体制也发生急剧变化，国家权力分散。1990年底，苏联解体。      

$\frac{a-1}{y-3}÷\frac{a^2-a}{y^2-9}$化简结果为（　　）

A：Hey, Mark! Have you decided which book to borrow from the library?

B：____ I will borrow Tom Sawyer.

A：That's an interesting book.

B：Really? ____

A：Yes, and I've read it twice。

B：Wow! ____

A：It's about an American boy. Many exciting things happened to him.

B：I see. ____

A：No. I've written about Oliver Twist.

B：____ I'm reading it now.