下面的因式分解，正确的是（　　）

A．x²-5x+6=（x-6）（x+1）

B．（1+xy）²-（x+y）²=（1+x）（1+y）（1+xy-x+y）

C．3x²-3x-6=（3x+3）（x-2）

D．ax²-6axy+9ay²=a（x-3y）²

短文改错。

     假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有

10处语言错误。每句最多两处错误。错误涉及一个单词的增加、删除或修改。

     增加：在缺词处加一个漏字符号(∧)，并在其下面写出该加的词。

     删除：把多余的词用斜线( \ )划掉。

     修改：在错的词下划一横线，并在该词下面写出修改正后的词。

     注意：每处错误及其修改均仅限一词；只允许修改10处，多者（从第11处起）不计分。

     Yesterday my friends or I visited a model high school. We start our visit from the main building. First we

went to the lab building there some students were doing experiments. Then we entered in the lecture hall

behind the lab building and saw many students listen to a lecture given a professor. Opposite to the lecture hall

was the art center. There they watched groups of students playing musical instrument. At the playground

some boys were playing the football. It was so exciting that we couldn't help cheering for them. It was really

a impressive visit.

_________________________________________________________________________________________