题目

On my first day of high school, going into math class, I was pointed and laughed at by two of my classmates. I initially thought my fly was open, or that something was stuck in my teeth. But as I took my seat, I heard one student whisper, “Why is a black boy taking Honors?” So, my fly wasn’t open. An honors level class had simply been taken by a student whose skin was brown.Many people think my clothes should be big enough for me to live in, or expect me to listen to only “black music.” In seventh grade, a group of my classmates fixed their cold stares on my clothes. They called out 设函数f（x）=x2-6x，则f（x）在x=0处的切线斜率为（　　）A．0B．-1C．3D．-6