题目

 I have to turn to my teacher for help, because this math problem is rather difficult______. A. worked out    B.to be worked out    C. working out    D. to work out 答案：D圆C1：x2+y2+2x+8y-8=0与圆C2：x2+y2-4x-4y-2=0的公共弦长等于 ．