兰兰第____名,冬冬第____名,婷婷第____名,丁丁第____名.

Dear Wang Tao,

    I'm so glad to read about the changes taking place in your city. I am happy that you can travel by subway in Chongqing, though there are only six lines there. Here is London we have twelve lines.

    The London Underground is very famous. It's famous for several reasons. Firstly, part of the underground railway today is the oldest in the world. It opened in 1863. Secondly, the London Underground is also the first line to have electric（电动的）trains in 1890. Thirdly, the London Underground is the second largest metro（地下铁道）system in the world, after the Shanghai Metro. There are 270 stations in the Underground and it has 402 kilometers of tracks (轨道). What's more, the city of London itself helps make the transportation（交通）system famous! I've got a few photos to help you build a clear picture of the Underground in my city. They are together with this mail.

    In your e-mail, you also said workers were building more lines. I'm sure your city is developing fast. Maybe you can send me a few photos of your city today. I really want to see the changes taking place there. I'm happy for you to live in an ancient city with such a new look!

    So much for now. I'll write soon again. Best wishes!

Yours,John

质量相等的A、B两球在光滑的水平面上沿同一条直线向同一方向运动，A球的动量是7kg•m/s，B球向动量是5kg•m/s，当A球追上B球发生碰撞，则碰撞后A、B两球的动量可能是（   ）