A:was holding
B:had held
C:was to hold
D:was to be held
读青藏地区图,回答下列问题.
如图,在等腰Rt△ABC中,∠A=90°,AC=7,点O在AC上,且AO=2,点P是AB上一动点,连接OP,将线段OP绕点O逆时针旋转90°,得到线段OD,要使点D恰好落在BC上,则AP的长等于____ .