题目

已知直线x+ay﹣1=0是圆C：x2+y2﹣4x﹣2y+1=0的对称轴，过点A（﹣4，a）作圆C的一条切线，切点为B，则|AB|=（ ）                                                                                                           A．2                            B．6                            C．4                       D．2如图，过⊙O外一点A引切线AB、AC，B、C为切点，若∠BAC=60°，BC=8cm，则⊙O的直径是    cm．