阅读下面文段回答问题

    There is a tiger and a monkey. They are friends. But one day they want to know who is stronger. One of them says, “The one who can get apples over there is stronger. ”There is a river over there. The monkey says, “I can't swim. ”The tiger says, “I can swim. Please sit on my back. ”They go across(穿过) the river. The apple trees are very tall. The tiger can't reach(够着)the apples. The monkey climbs up the tree and gets many apples. Now they know they should help each other.

《爱莲说》中点明了全文主旨的句子是____，____。

阅读理解

        大气中的气体容易散射紫、蓝、青三色光，所以一般情况下天空呈现蓝色。灰尘则不同，它不加选择地散射七色阳光。我们看到遥远的天空随高度降低而逐渐由蓝变白，就是因为底层大气中的灰尘含量较高。假如大气中没有灰尘，由于只存在气体对阳光的散射，整个天空将始终是蔚蓝色的。