阐述室外供热管网的敷设形式。

You have probably seen ads (广告) in newspaper or on television for mail-order companies. Perhaps a catalogue (商品目录) has been sent to you. Why do people buy things they have not seen in person Some people believe that things can be bought more cheaply by mail. Another good thing about buying by mail is that it is easier and more enjoyable to sit at home and work through a catalogue than to shop around the stores. With a catalogue from a large company, you have your own shop window for almost everything you want to buy.
Buying from a catalogue is so easy. It saves the shopper’s time and trouble. Sometimes, it saves the shopper’s money, too. But people often buy more than they can really afford(付得起) or need, because many things look so good to them. They can probably pay a certain part of the full price—a down payment. Then they pay a certain amount of money every month until the goods is paid for in full. But by then they often find what they have bought are of no use at all.

We know from the text that
[A] a catalogue may show you what a company is selling
[B] it is easier and more enjoyable to shop around stores
[C] you can set up your own shop with a catalogue