在地理上用字母表示经纬度，习惯上用S表示（　　）

阅读短文，判断正误。

Dear Joe,

How are you? My sister and I are having our holiday. Yesterday we went to the library. We got there by bus. We read storybooks and had lunch there. The food was delicious. It was cheap, too. We went shopping in the afternoon. I bought a pair of shoes. I was very tired. We are planning to learn skating and do many other things during the holiday. We are sure we'll have a good time.

How about your holiday? Please write to me soon.

Miss you.

Alice

设x,y满足约束条件$\left\{\begin{array}{c}2x+y-6\ge 0\\ x+2y-6\le 0\\ y\ge 0\end{array}\right.$则目标函数$z=x+y$的最大值是（  ）