如图所示，四边形ABCD、ABEF都是矩形，它们所在的平面互相垂直，AD=AF=1，AB=2，点M、N分别在它们的对角线AC、BF上，且CM=BN=a（0＜a＜$\sqrt{5}$），当MN的长最小时，a的值为（　　）

    The Noodle House has really good noodles. They are cheap, and they are great! You can get small, medium, or large bowls of noodles.

    The Noodle House has some specials. Special 1 has beef and tomato and is only ￥5. Special 2 has mutton and cabbage. It is ￥6. And Special 3 has chicken and carrot. It's only ￥8. I like it best.

    You can get some other great food at the Noodle House. For example, you can get 16 dumplings for ￥11. You can get great desserts! You can get strawberry, banana and orange ice cream.

    They are all nice! Try the Noodle House soon!

下列说法中正确的是（   ）