长城沿线易于相互嘹望的高岗丘阜之上建有大量的烽火台，它也是长城防御体系的重要组成部分，其作用是

What will the weather be like? Some people say they can know what the weather will be like from birds. When they see birds fly high out to the sea , they know it is a nice day, because birds don't like to fly out when it will rain. When the weather will be bad, the birds come back to the beach. A big wind may come with a rain. Birds do not like to fly in a big wind. They may sit in a tree when rain is coming. Other animals can also tell the weather. It's a cloudy day. The rain is coming. Chickens aren't quiet, and they are running here and there. Frogs(青蛙) are making big noise in the pool. If it is a nice day, the frogs will come out of the water.

小阳发现电线杆AB的影子落在土坡的破面CD和地面BC上，量得CD=8米，BC=20米，CD与地面成30º 角，且此时测得1米杆的影长为2米，则 电线杆的高度为（       ）
     

补写出下列句子中的空缺部分。

①在《论语》中，孔子用“____，____”两句表明了“思”与“学”的关系。

②《庄子·逍遥游》中以“朝菌”和“蟪蛄”为例来说明“小年”一词的两句是“____，____”。

③在《陈情表》中，李密以乌鸟反哺作比表达孝道的句子是“____，____”。

④陶渊明在《归去来兮辞》中“____，____”两句形成了鲜明的对比，即精神上的富足、孤高与物质生活的清贫之间的对比。