设集合A={1，2，3}，B={2，4}，则A∩B=____ 

己知：

(C)

    If someone asks you, "What colour is the sky?" I think the answer will be "blue". I'm afraid that you are wrong. Because the sky has no colour.

    What's in the sky? Is the sky full of air? I'm sure that you have asked these questions. We all know that there's air all around the world. We can not live without air. Planes can't fly very high because as they go higher, the air gets thinner. If we go far enough away from the earth, we can find that there is no air. The sky is space（太空）. In this space there is nothing but the sun, the moon and the stars.

    People always want to know more about other worlds in space. They have looked at them through （通过） telescopes （望远镜） and in this way they have known much. They know a lot about the moon now. But they want to know more about space. I'm sure they will go farther and farther until they know all about it.

____的中间有一条比较粗壮的根，这条粗根周围有许多细小的根，这类根是____；____的根是由许多粗细差不多的根构成的，中间没有明显的粗根，这类根是____；