听力原文： Let' s proceed to the main exhibition hall and look at some of the actual vehicl

es that played a prominent role in speeding up mail delivery. Consider how long it used to take to send a letter across a relatively short distance. Back in the 1600s, it took two weeks on horseback to get a letter from Boston to New York, a distance of about 260 miles. Crossing a river was also a challenge. Ferry services were so irregular that a carrier would sometimes have to wait hours just to catch a ferry. For journeys, inland there was always a stagecoach but the ride was by no means comfortable because it had to be shared with other passengers. The post office was pretty ingenious about some words. In the 19th century, in the southwestern desert, for instance, camels were brought in to help to get the mail through. In Alaska, reindeer were used. This practice was discontinued because of the disagreeable temper of these animals. We' ll stop here a minute so that you can enter this replica of a railway mail car. It was during the age of the iron horse that delivery really started to pick up. In fact, the United States transported most bulk mail by train for nearly 100 years. The first airmail service didn' t start until 1918. Please take a few moments to look around. I hope you' ll enjoy your tour. And as you continue on your own, may I suggest you visit our impressive philatelic collection？ Not only can you look at some of the more unusual stamps issued but there' s an interesting exhibition on how stamps are made.

What is the talk mainly about？

A．Overland transportation in the nineteenth century.

B．Historical aspects of mail delivery.

C．Vehicles currently in use by the postal service.

D．The invention of railroad.