题目

 Determined not to fall behind, Alex now spends ________ as he used to on physics, his weakest subject. A. twice as much time    B. as twice much time C. twice much time as    D. as twice time much 答案：A如图，AB是⊙O的直径，BC，CD，DA是⊙O的弦，且BC=CD=DA，则∠BCD=________．