（ $1$ ）求椭圆的方程．

已知等差数列{a_n）的前n项和为S_n=﹣n²+（10+k）n+（k﹣1），则实数k=____ ，a_n=____ 

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    Tolstoy, the great Russian writer, liked to walk about in a railway station near his home. One day when he was walking up and down as usual, looking at people getting on and off the 1, he heard a lady 2 after him, “Hey, you old fellow, go and fetch my handbag in the 3  room which I left there.”

    Tolstoy 4 there. He 5 the bag up and walked quickly along the platform (站台）. At the same time the lady was waiting beside the carriage, looking  6. When at last the old man gave the bag back to her, she opened it to 7 sure nothing was missing. “Good, old man,” said the woman. “You are just as 8 as I hope. Here you are.” Tolstoy 9 the coin and put it into his pocket with a smile.

    But the woman was very 10 when she heard that he was Tolstoy, the author (作者）of the great novel WAR AND PEACE. How interesting it was!

已知点P在直线x+3y﹣2=0上，点Q在直线x+3y+6=0上，线段PQ的中点为M（x₀，y₀），且y₀＜x₀+2，则$\frac{y_0}{x_0}$的取值范围是（　　）