解:(1)证明:证法一:选①BE =DF "
在□ABCD中 , AB = CD, AB//CD, ∴ ∠ABE = ∠CDF
又∵ BE=DF, ∴△ABE≌△CDF AE= CF
同理AF= CE
∴四边形AECF是平行四边形
证法二:选③∠AEB= ∠CFD, ∵在口ABCD中,AB=CD,AB//CD
∴∠ABE =∠CDF
∵∠AEB =∠CFD, ∴△ABE≌△CDF ∴AE= CF
又∵∠AEB=∠CFD, ∴∠AEF =∠CFE
∴ AE//CF
∴四边形AECF是平行四边形
(2)
A. AB=CD,AD=BC
B. AB=CD,AD//BC
C. AB//CD,AB=CD
D. AB//CD,AD//BC
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