Have you ever seen a horse with toes? Millions of years ago, horses had many toes. They had four toes on each front foot. They had three toes on each back foot. The horses were no bigger than the cats. These small horses lived in the forest. The toes helped the horses run on the soft, wet ground.

    At first, it was very hot in the forest. But the weather changed. It became very cold. Many trees could not live in cold weather. The trees died and fell. Open fields took the place of forests. The sun made the ground dry and hard.

    Horses began to change, too. They began to get bigger. This took a long time. On the dry, hard land, horses needed only their middle toes for running. Their middle toes became hard. After a long time, horses had only one hard toe on each foot. Now we call this hard toe a hoof(蹄子).

1966年7月1日，中共中央、中央军委做出决定，组建战略导弹部队，直接领导和指挥这支部队的是（    ）

先化简，再求值：（1﹣$\frac{1}{x+2}$）÷$\frac{x^2+2x+1}{x^2-4}$，其中x=4．