补全下面的短文。
    Last Friday，John went on a school trip with his classmates．They____the science museum．John learned a lot about robots．Some of them____sing and dance．They were so cool John____lots of photos there．After that，he went to the gift shop and____some lovely gifts for his parents．They____cheap．

已知，则                                 （   ）

下列不存在丁达尔效应的分散系是                                

课外阅读鲁迅的小说《非攻》，比较一下，看看小说中的墨子和《公输》中的墨子形象有什么不同。

棱柱的侧棱