如图，已知Rt△ABC中，∠ACB=90°，D是AB的中点，AB=5，则CD=____．

生产上培育无子番茄、青霉素高产菌株、杂交培育矮秆抗锈病小麦、抗虫棉的培育原理依次是（）
①生长素促进果实发育   ②染色体变异   ③基因重组 ④基因突变 ⑤基因工程

在我国，回族和维吾尔族信仰的宗教是（　　）

高速公路上对不同类型的车辆有不同的限速规定，这样做可以控制汽车行驶时的（   ）

    It seems to be strange to you there is a blind spot(点) on the eyes. Here is an interesting experiment that can make something disappear, when one eye is open.
　Make a card about the size of a postcard and write two English letters L and R on it, L on the left and R on the right. First, hold the card about 80 cm away and you see both the letters. Then close your right eye and look at the letter R only with your left eye. And now, as you move the card slowly towards you, you'll find the letter L disappearing. But if you move the card nearer to your face, the letter will be seen again. Now do the same experiment with your left eye closed, you'll find the letter R disappearing.
    Why does the letter disappear? It is because there is a blind spot on the eye. When the image of the letter falls on the blind spot, it won't be seen. That is why either of the letters disappears.