I'm a middle school student from America. I live far away from my school. So I often go to school by bus. It takes me about twenty minutes from my home to school.

    But at my school and other schools in our town, there is a "Walk to School Day". It is on Wednesday every week. On that day, parents send their kids to a bus stop. There are many bus stops in our town. At each bus stop, two adults lead us to walk to school together.

    "Walk to School Day" stops us taking cars or buses. That is good for the air and we can get exercise too. On the way to school, we talk to each other. We can also make new friends. I like "Walk to School Day". I walk about five kilometers with other students on that day. Our school will give some students beautiful backpacks if they do well. And I got one last year.

根据汉语意思完成英语句子，每空一词。

圆$x^2+y^2=1$与直线$y=kx-3$有公共点的充分不必要条件是 （     ）