找出下列选项中与其他两个不同类的单词()
A:China
B:UK
C:USA
因式分解:x2﹣9=____.
阅读理解
Name: Kelsey
Age: 22
I can sing and dance. I can play the violin and the piano, but I can't play the guitar.
Name: Paul
Age: 13
I can sing, but I can't dance. I can play the trumpet and the guitar but I can't play the violin.
如图表示的是某种物质在血管中含量变化情况,如果a代表肺泡处的毛细血管,b代表组织细胞处的毛细血管,则此物质是( )