David is from America. He and his family____(have) different plans for the coming holiday. David likes China very much and he____ (go) to Beijing last year. But he wants to know more about Chinese culture so he____ (visit) Beijing again. This time he is going to the Palace Museum. There____ (be) many interesting things about Chinese culture there. He____ (want) to take lots of photos of the things in the Palace Museum. He____ (show) them to his friends. David's father ____ (read) many books about Japanese history three months ago. He thought some things in Japan were interesting. He____ (take) a trip to Japan. David's mother didn't know where to go at first. At last she ____ (decide) to go to Singapore. She likes the climate (气候) there. What about David's sister? She ____ (stay) at home and do her homework.

下列常见的物质中，属于纯净物的是 （   ）

春秋战国时期，社会急剧变化，形成了“百家争鸣”的学术繁荣局面。其中，“学而时习之，不亦说乎”出自（    ）