一组数据4，5，6，7，7，8的中位数和众数分别是（  ）

①当a=1时，方程组的解也是方程x+y=2的一个解；

②当x﹣2y＞8时，a＞ $\frac{1}{5}$ ；

③不论a取什么实数，2x+y的值始终不变；

④某直角三角形的两条直角边长分别为x+y，x﹣y，则其面积最大值为 $\frac{8}{3}$ ．

以上说法正确的是（   ）

Dear Amy,

    Thanks for your letter. It's always nice to hear from you.

    Today is the last day of my exams. The maths exam is very difficult, but other exams are easy. I'mwaiting for the holidays now, and I'm very anxious to see you and to be with you again. I'm going to spend my Christmas and New Year holidays in New York with my sister. I'm going down town. I'm going to go shopping for Christmas gifts. I'm excited about Christmas, and about seeing you again.

                                                                                                                                                                                                        Yours,

                                                                                                                                                                                                           Jack