Every day in China, about 200 million children go to school. Many of them take school buses to school and back home. Most of the time, the school buses are very safe for them. But sometimes they are also dangerous.

    Several years ago, a serious accident happened to a nine – seat minibus in November in Gansu. There were 62 children inside, and 21 of them died.

    What should we do to keep us safe on the road? First, we should wait for the school bus on time at the right place when we are going to school or coming back home. Second, we shouldn't run or fight on the bus when it is going. Third, we mustn't try to get off when it is moving. Last, don't get on the bus if it is overloaded(超载).

    Our government(政府)also makes rules for school buses. Under the new rules, school buses have a speed limit of 80km per(每)hour on highways and 60km per hour on normal roads. While on the road, other cars must let school buses go first.

根据材料内容选择最佳答案。

在平面直角坐标系xoy中，圆C的方程为$x^2+y^2-8x+15=0$，若直线y=kx-2上至少存在一点，使得以该点为圆心，1为半径的圆与圆C有公共点，则k的最大值为