如图，AB∥CD,AD与BC相交于O，那么下列比例式正确的是  (    )

A：$\frac{AB}{CD}=\frac{OA}{AD}$

B：$\frac{OB}{OD}=\frac{OA}{OC}$

C：$\frac{OB}{OD}=\frac{BC}{AD}$

D：$\frac{AB}{CD}=\frac{OB}{OC}$