题目

Around the world there are some festivals that for some of us may sound bizarre. In England they celebrate the rolling cheese race, which occurs every year during the last Monday in May. This festival’s characteristic is a contest where an official rolls cheese down a steep hill and festival- goers go chasing after it. The winner is the first one to reach and catch the cheese. Another festival celebrated in England is the world championship of grimaces (鬼脸). It is a celebration that dates back to 1297 and is celebrated during the month of September. The task is to perform the bes抛物线的一组斜率为2的平行弦中点的轨迹是（     ）A．椭圆B．圆C．双曲线D．射线(不含端点)