题目

 It was playing video games that took the boy too much time that he _______ have spent learning．     A. could            B. must                 C. might           D. ought to 答案：D11．如图，在Rt△ABC中，∠ACB=90°，∠BAC=30°．分别以直角边AC和斜边AB向外作等边△ACD、等边△ABE．过点E，作EF⊥AB，垂足为F，连结DF．求证：（1）AC=EF；（2）四边形ADFE是平行四边形．