根据短文内容，完成下面的任务。

Dear Grandma,

    How are you? I am very busy this month. (A)September 5th is my good friend Gina's birthday. I will go to her birthday party in the evening. On the eleventh, (B)我有一个英语测试. On September 17th, I have a baseball game. You know I love baseball. On the 24th, I have a school trip. On the 28th, I'll go to Mr. Cool's Clothes Store. The clothes there are on sale that day. I want to buy a pair of sports shoes and a T-shirt. The hats there are very nice. I will buy one ① ________ you. Oh, Grandma, I'm very sorry I can't go to see you ② ________ my parents this month. See you next month.

 Yours,

Eric

关于x的方程组$\left\{\begin{array}{c}3x-y=m\\ x+my=n\end{array}\right.$的解是$\left\{\begin{array}{c}x=1\\ y=1\end{array}\right.$，则$\sqrt{m^2-n}$的值是（   ）

如右图所示，物体静止于光滑水平面M上，水平恒力F₁作用于物体，现要使物体沿着OO′方向做直线运动（F₁和OO′都在M平面内）．那么必须同时再加一个力F₂，则F₂的最小值是（   ）