题目

如图，已知AB，CD相交于点O，AC∥BD，＝，CO＝6，则DO＝( )A. 21 B. 15 C. 9 D. 5 答案：【答案】B【解析】由AC∥BD，易证△AOC∽△BOD，得＝，结合已知条件，则可求DO的值.解：∵AC∥BD∴∠C＝∠D，∠A＝∠B∴△AOC∽△BOD∴＝，∵＝，CO＝6，∴DO＝15故选：B．— I can’t open this door, Jack. Could you give me a hand?—_________.A. Thank you a lot. B. Here you are.C. That’s all right. D. Of course.