题目

If we can _____ our present difficulties, then everything should be all right.A. get off      B. come across      C. come over    D. get over   答案：Dget over (difficulties)克服(困难)。其它几组短语不符题意。如图，AB是⊙O的直径，AM、BN分别切⊙O于点A、B，CD交AM，BN于点D、C，DO平分∠ADC．（1）求证：CD是⊙O的切线；（2）若AD=4，BC=9，求⊙O的半径R．