听力原文：In the past, messengers often carried letters from place to place. In Europe, it

was not until the 15th century that a regular system of postal service between towns was started. The messengers traveled on horseback, and fresh horses were always kept ready at different places along the main roads where the messengers stopped. Each of these stops was known as a post and today we still keep the name "Post office".

In the old days, such postal deliveries were only for important people. The messengers would sometimes carry letters for ordinary citizens, but the cost was very high and the messengers were often careless. And so in the 17th century in Britain, a service was started for everyone. At first this postal service carried letters only between a few very busy towns, but year by year it grew larger and better.

Today, there are post offices in every country. Letters can be sent by post in almost any village, anywhere. In Britain, the postman who delivers the mail may walk or ride bicycle for a short distance. Post offices also use many other ways to carry mail — by train and ship and by airplane, depending on the fastest and easiest way.

(23)

A．In 15th century.

B．Before 15th century.

C．In the 17th century.

D．After 17th century.