如图,AB是⊙O的直径,弦CD垂直平分OB,则∠BAC等于( )
A:15°
B:20°
C:30°
D:45°
A.
B.
C.
D.
E.
They are doing their homework.(对划线部分提问)____
两极格局彻底瓦解的标志是( )