题目

Where is my book? A . It's under the bag. B . It's big. 答案：A20．将四根长度相等的细木条首尾相接，用钉子钉成四边形ABCD，转动这个四边形，使它形状改变，当∠C=90°时，测得AC=2$\sqrt{2}$，当∠C=120°时，如图2，AC=（　　）A．2B．$\frac{\sqrt{2}}{2}$C．$\frac{\sqrt{3}}{2}$D．$\sqrt{2}$