现有H、S、O、Na四种元素，请选用其中的元素写出符合下列要求的化学式各一个：
题型：填空题
问题：
现有H、S、O、Na四种元素，请选用其中的元素写出符合下列要求的化学式各一个：
（1）相对分子质量最小的氧化物_________；
（2）用于金属表面除锈的酸____________；
（3）厨房清洁剂中含有的碱_____________；
（4）由三种元素组成的一种盐__________。
题型：填空题
阅读理解。
American schools’ beginning is in September after a long summer holiday. There are two terms in a
school year. The first term is from September to January, and the second is from February to June. Most American children begin to go to school when they are five years old. Most students are seventeen or
eighteen years old when they finish high school.
High school students take only four or five subjects each term. They usually go to the same classes e
very day, and they have homework for every class. After class they do many interesting things.
After high school, many students go to college. They can go to a small one or a large one. They usually
have to give a lot of money. So many college students work after class to get the money for their studies.
1. How many terms are there in a school year in America ?
A. Four
B. One
C. Three
D. Two
2. The first term is ______.
A. from February to June
B. from January to September
C. from June to February
D. from September to January
3. When a child is ______ years old he or she can go to school.
A. five
B. four
C. seven
D. six
4.How many subjects do the high school students take each term ?
A. As many as they like.
B. Five. 
C. Four.
D. Four or five.
5. Why do many college students have to work after class ?
A. Because some work is helpful to their studies.
B. Because the teachers ask them to do so.
C. Because they like to work very much.
D. Because they need to get money for their studies.
热分解法，Ⅱ、热还原法，Ⅲ、电解法，Ⅳ、湿法冶金。下表中金属与常用冶炼方法相符的是： 