A:①②③
B:①②③④
C:②③④
D:①③④
生物分类等级中,介于科和种之间的分类单位是( )
Has he ____(安排) for a car to pick them up from the airport?
如图,点A、B在⊙O上,直线AC是⊙O的切线,OD⊥OB,连接AB交OC于点D.
(1)求证:AC=CD;
(2)若AC=2,AO=5,求OD的长度.