题目

— There are always many volunteers in great events, aren't there?—Yes. ________． A . No pain, no gain B . Many hands make light work C . Actions speak louder than words D . A friend in need is a friend indeed 答案：B如图，已知⊙O的半径为R，AB是⊙O的直径，D是AB延长线上一点，DC是⊙O的切线，C是切点，连接AC，若∠CAB=30°，则BD的长为    ．