题目

    A group of swans（天鹅） 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（特技）, and then came down and looked proudly at the s已知抛物线y2=4x的焦点为F，准线与x轴的交点为M，N为抛物线上的一点，且|NF|=|MN|，则∠NMF=（ ）A．B．C．D．