用简便方法计算（写出主要的简算过程．）
                           

在□里填上合适的数。

甲、乙两辆汽车在平直的公路上沿同一方向做直线运动，t＝0时刻同时经过公路旁的同一个路标．在描述两车运动的v－t图中(如图4所示)，直线a、b分别描述了甲、乙两车在0～20秒的运动情况．关于两车之间的位置关系，下列说法正确的是

先通读下面的短文，掌握其大意，然后从A、B、C、D四个选项中选择可以填入空白处的最佳答案。
    Once there was an old woman who liked to make cookies(曲奇饼).She didn't have1to share them with，so she always gave them away to the children in the neighborhood.This continued for several years and the cookies became quite famous，2at Christmas time.The parents began to like the cookies as much as the children did，and the old woman's kitchen became very busy, so the children came to3.
Then one Christmas the old woman became ill.There were no cookies，and there was no money to pay the doctor.The children were very4.Later they had a(n)5 ， that is，to make cookies from the old woman's recipe(食谱).So the children 6a cookie store.Soon they had a very good cookie business and got7money to Pay the doctor.
    After the old woman got well，she was asked to become the manager(经理)of the store，selling special cookies8all the holidays of the year.9the old woman died，the children，now grown up，kept the cookie business alive(有生气的)in her name.
    Maybe life is like a Christmas cookie.The more you10it with your friends, the more you may enjoy it.

已知点O（0，0），A（4，0），B（0，3）为矩形的三个顶点，求矩形的两条对角线所在的直线的方程．