阅读下面的文本, 从A、B、C、D四个选项中选出最佳选项。
My friend Bob has a large police dog named Jack. Every Sunday afternoon, Bob and Jack have a walk in the park near their home. Jack likes these walks very much.
On a cold Sunday afternoon, I go to Bob's home. Bob is happy to see me. At first Jack is happy too. Bob and I talk for a long time. Jack plays with a ball in the room for some time. Then he sits down in front of me and looks at me. I go on talking with Bob. Just at that time, Jack takes my hat and runs out of the room. I go after him to get my hat back. Suddenly, I know what Jack wants to do.
What a clever dog!
满足：
如果
为数列
的前n项和，那么
的概率为