图中的黑色部分表示四种动物共有的特点，则该特点是（　　）

已知f（x）=ax²+bx+1是定义在﹣2a，a²﹣3上的偶函数，那么a+b的值是（  ）

阅读理解，判断正误。

    A dog is walking along a road. It is a big dog. It is carrying a piece of meat. The piece of meat is in the dog's mouth. The dog is walking across a bridge now. There is water under the bridge. The water is very deep. The dog is looking into the water. It can see a dog in the water. This dog has a piece of meat in its mouth, too. The big dog is barking at the dog in the water. It wants its piece of meat, too. The big dog's piece of meat is falling into the water.

如图所示，为测量作匀加速直线运动小车的加速度，将宽度很小且均为b的挡光片A，B固定在小车上，测得二者间距为d（d远大于b）；当小车匀加速经过固定在车旁的光电门（图中没有画出）时，测得两挡光片先后经过光电门的时间分别为和t₁和t₂，则小车的加速度 a=____