将绿矾(FeSO₄·7H₂O)、硫酸铵以相等物质的量混合可制得莫尔盐晶体。(NH₄)₂SO₄＋FeSO₄＋6H₂O= (NH₄)₂SO₄·FeSO₄·6H₂O↓，根据下图回答：

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    I have a good friend. 1 Chinese name is Da Shan and his 2 name is Mark Rowswell. He 3 from Canada. He can 4 English and French (法语). And he speaks Chinese very 5. He likes China very much. Now he teaches English 6 Beijing. Da Shan 7 a big nose, big eyes 8 short brown hair. He has a happy family. His wife (妻子) is from Sichuan, China. They 9 two children. Da Shan's father and mother are in Canada. But 10 like China, too. And they often come to

China.

收音机的音量开关是一个电位器，它是由一个金属片在圆滑形碳膜电阻上滑动而组成，用久了的收音机在旋转音量开关时，往往会听到很响的“喀喀”杂声，这是碳膜严重磨损或电位器中有灰尘的缘故，请解释这种现象．