题目

First Lady Michelle Obama is a big fan of volunteering. Volunteering means working for free to help someone else. Mrs. Obama says volunteering is very important. “It should be part of everyone’s life,” she says. Many teens agree. They say that helping others feels great and makes a difference. These days, more teens volunteer than work for pay. Teens clean up parks, walk dogs at animal shelters, visit the elderly and more. Some cities — including Seattle, Chicago, and Washington, D.C. —require high school students to volunteer. Students must volunteer in order to graduate. Th15．设变量x，y满足约束条件$\left\{\begin{array}{l}{x-y+2≥0}\\{2x-y-2≤0}\\{x+y-2≥0}\end{array}\right.$，则z=3x-y的最大值为（　　）A．-2B．$\frac{10}{3}$C．6D．14