阅读下面短文并回答问题
[1]Stamp collectors usually collect stamps only from certain countries or regions. The traditional method of stamp collecting is to organize stamp collection according to the issue dates.
[2]Of course, there are many ____________. You can collect stamps relating to a particular theme. For example, sport lovers may choose the Olympic theme. They try their best to find Olympic stamps issued by various parts of the world every four years. Some of them may want to concentrate on even more specific sport activities like ball games, gymnasium, swimming, etc.
[3]If you like astronomy, you may be keen on collecting stamps describing the stars, planets, spacecraft, spacemen, etc. once you have fixed the topic, you no longer need to bother about stamps other than the theme you selected. Then, you can put all your effort to collect the stamps around it.
[4]China issues zodiac animal stamps (十二生肖邮票) every year to celebrate the Lunar New Year. They are extremely popular and a lot of collectors wait patiently for 12 years to collect a full set of them. Hong Kong’s Lunar New Year special stamp series began in 2000 with the Year of Dragon and the twelfth and final set was issued in 2011, the Year of Rabbit. Recently some Western countries have issued zodiac animal stamps, which attract a lot of attention as well.
[5]You may choose any theme you like. However, you should bear in mind an essential point. The topic you choose should not be too narrow. Otherwise, it is very difficult to accumulate stamps around the topic. You will be depressed and even lose your interest in collecting stamps.
[6]Collecting stamps with themes is especially popular among teenagers because there is a wide range of choices for them. The main challenge is how you organize and expand your collection around your theme.
What is the main idea of the passage? （no more than 5 words）
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Fill in the blanks in Paragraph 2 with proper words． （no more than 5 words）
Of course, there are many ____________________________________________.
What should you do if you want to collect a full set of zodiac animal stamps? （no more than 7 words）
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List the three general themes in collecting stamps mentioned in the passage． （3 words）
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What does the word "them" （Line2, Paragraph6） probably refer to? （1 word）
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