题目

阅读理解 A group of swans flew down to a beach where a crow(乌鸦) was jumping around. The crow watched them with disdain(鄙视). “You have no flying skills at all!” he said to the swans. “All you can do is to move your wings. Can you turn over in the air? No, that's beyond you. Let's have a flying competition. I'll show you what real flying is!” One of the swans, a strong young male, took up the challenge. The crow flew up and began to show his skills. He flew in circles, performed other flying tricks, and then came down and looked proudly at the swan. Now it was the swan's turn在空间四边形中，分别是的中点。若，且与所成的角为，则四边形的面积为(    )A．B．C．D．