题目

If the windows are shut, air can't (流动) freely through the building. 答案：【1】flow已知两圆的方程分别为x2+y2=1和x2+y2-6x-8y+9=0,那么这两个圆的位置关系是(    )A.相离               B.相交                C.内切                D.外切