【正确答案】 A

【答案解析】[听力原文 18-20] 　　The first postal service in North America began in New England in the 17th century. All mail arriving in Massachusetts colony was sent to the home of the appointed official in Boston. In turn, he would deliver the mail from Boston on horseback to its destination, receiving one penny for each good article of mail. Later in the century postal services were established between Philadelphia and Delaware. In 1691, the British crown appointed the first postmaster general to have charge of the mail for all the colonies in North America. Later, Benjamin Franklin served as the postmaster general for the British government and then was made postmaster by the newly formed United States government. Franklin was responsible for establishing the United States postal system on permanent basis. He increased the number of post offices, introduced the use of stagecoaches to carry mail, and started a package service system. Later, in the nineteenth century, as railroad and steam boats appeared, they were used to carry mail into the towns. Some communities, especially those out west, were far from the services of transportation. To serve them, the post office developed a system called "star routes". Private contractors paid to deliver mail to the communities from railways by horse and wagon. The postal service, which was started over 3 centuries ago, has developed into an extensive government service with post offices in every city, town, and village in the United States. [解析]文中有一句是这样说的：Some communities，especially those out west，were far from the services of transportation．To serve them，the post office developed a system called“star routes”．在这句话中，不定式短语 to serve them 用作目的状语解释了开通 star routes 的原因。