A:①②
B:②③
C:①③
D:①④
Only by constant study and changing our ideas can we________ to a changeable environment.
如图,已知⊙O的半径为2,C为直径AB延长线上一点,BC=2.过C任作一直线l.若l上总存在点P,使过P所作的⊙O的两切线互相垂直,则∠ACP的最大值等于____ .