When you post a letter, you must buy ______.

A．Grown - ups like new stamps while young boys like used ones.
B．There are hundreds of kinds of stamps in the world.
C．Sometimes stamp collectors are willing to pay thousands of yuan or more for a used stamp.
D．A post - office worker does not want to buy any used stamps.

A．Some stamps have a flower or animal on one side.
B．Along with a picture on one side, there are also some words.
C．Some used stamps can be worth a lot of money.
D．A stamp, when used, is worth more than a new on