A:to buy
B:which to buy
C:for him to buy
D:with which to buy
____×____=____
____÷____=____
① OA=OC ② AB=CD ③ ∠BAD=∠DCB ④ AD∥BC
请你从中选择两个论断作为条件,以“四边形ABCD为平行四边形”作为结论,完成下列各题: