题目

While it is impossible to live completely free of stress，it is possible to prevent stress as well as reduce its effect when it can’t be avoided．The US Department of Health and Human Services offers the following suggestions for ways to deal with stress．   ◆Try physical activity   When you are nervous，angry or upset，try releasing the pressure through exercise or physical activity．Running，walking，playing tennis，and working in your garden are just some of the activities you might try．   ◆Take care of yourself   You should make every effort to eat well and get enough 已知x+y=4，xy=3，那么x2-xy+y2的值为（　　）A．7B．10C．13D．16