题目

— You may find the key to the maths problem on page 107. — Ah, it’s so simple. I wonder why I         of that.           A. hadn’t thought                             B. haven’t thought                      C. didn’t think                                  D. wasn’t thinking 答案：C在△ABC中，∠C=90°，AC=BC，AD平分∠BAC，DE⊥AB于E，若AB=20㎝，则△DBE的周长为  .