Read the passage and choose the best answer to each question. Stamp collecting is the collecting of postage stamps and related objects, such as overs (envelopes or packages with stamps on them). It is one of the world’s most popular hobbies, with estimates of the number of collectors ranging up to 20 million in the United States alone. Many casual collectors enjoy accumulating stamps without worrying about the tiny details, but the creation of a large or comprehensive collection generally ...

Read the passage and choose the best answer to each question.

Stamp collecting is the collecting of postage stamps and related objects, such as overs (envelopes or packages with stamps on them). It is one of the world’s most popular hobbies, with estimates of the number of collectors ranging up to 20 million in the United States alone.

Many casual collectors enjoy accumulating stamps without worrying about the tiny details, but the creation of a large or comprehensive collection generally requires some philatelic knowledge. This is especially important for those who intend to spend large amounts for stamps.

Stamp collectors are an important source of revenue for some small countries who create limited runs of elaborate stamps designed mainly to be bought by stamp collectors. The stamps produced by these countries far exceed the postal needs of the countries.

Some collectors, observing the generally rising prices of rare stamps, have taken to Philatelic Investment. Rare stamps are among the most portable of tangible investments, and are easy to store. They offer an attractive alternative to art, other collectible investments, and precious metals.

The word they in paragraph 4 refers to ……….