题目

China, a country with 145 million elderly people, will remain _______ ageing society during this century, according to _______ Chinese population experts. A. /; the                        B. an; /                        C. an; the                            D. the; / 答案：B如图，点A、B在⊙O上，直线AC是⊙O的切线，OD⊥OB，连接AB交OC于点D．（1）求证：AC=CD；（2）若AC=2，AO=，求OD的长度．