题目

Alan is a careful driver, but he drives ______ of my friends. A. more carefully   B. the most carefully   C. less carefully      D. the least carefully 答案：D如图，AB为的直径，C为上一点，D为BA延长线上一点，．求证：DC为的切线；线段DF分别交AC，BC于点E，F且，的半径为5，，求CF的长．