一道prep老数学题

一床空悬

这是第二次做这道题了，第一次毫不犹豫选了D。
这次却犹豫不决，是怎么得到ABC共线的（因为在推理过程中需要用到∠CBO是△ABO的外角）？或者这个思路本身就错了？
为什么突然钻到了牛角尖里.....




In the figure shown, point O is the center of the semicircle and points B, C, and D lie on the semicircle.
If the length of line segment AB is equal to the length of line segment OC, what is the degree measure of angle BAO ?

(1) The degree measure of angle COD is 60.

(2) The degree measure of angle BCO is 40.