一杯食盐溶液，从溶液的上部取出一些，测得它的密度为a g/cm³；再从下部取出一些，测得它的密度为b g/cm³，则a与b的关系正确的是(    )

用方框中所给单词的适当形式填空。

         As we know, there are many beautiful places in China, such as Hong Kong, Harbin. They attract a lot of people at home and abroad. But it's hard for ____ to choose where they can go. You can tell them ____ Guide to China, which ____ China in detail. If they have no chance to go through the book, you may show them advertisements for traveling, then after ____, they can decide where to go. They will have a good trip and take photos in ____ of it.

如图，如果∠AOB=∠COD=90°，那么∠1=∠2，这是根据（　　）

下列历史事件按照时间顺序排列正确的是（    ）

①忽必烈继承汗位 ②蒙古政权建立 ③忽必烈定都 ④元朝建立

读下图，回答下列各题。