根据情景和所给的首字母，在空格内填入一个适当的单词，使对话意思完整

What must you do when you get a present for your birthday? You have to sit down and

w  1  a thank-note. The w 2   “thank you” are very important. We have to use them very often. We say them when someone gives us a drink, h  3   us to pick up things, or lends us a book.

A  4    important word is “please”. Many people forget to use it. It is n  5    polite to ask someone to do something without s  6    “please”. We have to use it when we ask for something, too. It may be a book or a pencil, more rice or help.

We have to l  7    to say “sorry”, too. When we hurt someone, we’ll have to go up to say “we’re sorry”. When we forget something, we will a  8    say “sorry”.

These three words are easy b  9    important. Our children must learn to use them. They are pleasant words to use in a  10    language.

空气中有哪些杂质？在空气分离过程中为什么要清除杂质？