题目

It was an exciting moment for these fans this year, _____ for the first time in years they saw their favorite singers.    A. that    B. which    C. when   D. while 答案：C 解析: 略已知：如图，在△ABC中，点D是∠BAC的角平分线上一点，BD⊥AD于点D，过点D作DE∥AC交AB于点E．求证：点E是过A，B，D三点的圆的圆心．