题目

 UK Prime Minister Cameron has become the first world leader ______ to fly onboard Air Force One with US Prsident Obama.    A. inviting                B. invited            C. being invited        D. having invited 答案：B平面直角坐标系中，⊙M的圆心坐标为（0，2），半径为1，点N在x轴的正半轴上，如果以点N为圆心，半径为4的⊙N与⊙M相切，则圆心N的坐标为________．