题目

Having achieved his goal has given him muchconfidence, and he works even harder. 答案：实现的目标给了他很大的信心，他工作更努力了。如图，AB是⊙O的直径，点C在AB的延长线上，CD切⊙O于点D，过点D作DF⊥AB于点E，交⊙O于点F，OE=1cm，DF=2cm，则CB的长为（ ）A．B．C．D．4