题目

That woman carried a new blanket (毛毯) over her arm. Wordlessly, she gave it to me. “Is it finished?” I asked. She shook her head. “No. It is ready,” she replied. I handed her the money and took the blanket. “It is beautiful, so skillfully woven (编织),” I said to my mother. “But what did she mean when she said it was not finished? How can it be ready if it is not finished?” “I will tell you later,” my mother said, “but first I will take you to the Navajo(纳瓦霍) village.” We went down to the village. A group of young men were making sand pictures. 如图，请在下列四个关系中，选出两个恰当的关系作为条件，推出四边形ABCD是平行四边形，并予以证明．（写出一种即可）关系：①AD∥BC，②AB=CD，③∠A=∠C，④∠B+∠C=180°．已知：在四边形ABCD中，　　　　　　，　　　　　　；求证：四边形ABCD是平行四边形．