如图所示，有关这个小小蒸汽轮机的说法不正确的是（    ）

①a²﹣b²；②a（a﹣b）+b（a﹣b）；③（a+b）（a﹣b）； ④（a﹣b）²．

其中正确的表示方法有（   ）

Dear David,

    I'm very glad you are arriving in my city at 3:00 p.m. this Sunday. Now let me tell you the way to the Blue Sky Restaurant. I'll meet you there. Take a taxi from the bus station and go down New Bridge Avenue. Go across the big bridge. When you see a bank, turn right and go through Bank Street. You'll pass three one-way avenues: Sixth Avenue, Seventh Avenue and Eighth Avenue. When you see a big McDonald's, turn left. Then go along Green Avenue until（直到）you see New Park. Turn left and go down Center Street. The Blue Sky Restaurant is on your right.

    I hope you have a good trip.

Yours,

Larry