题目内容：
根据材料，回答题。 The Development of American Postal System
In the early days of the United States, postal charges were paid by the recipient and chargesvaried with the distance carried. In 1825, the United States Congress permitted local postmastersto give letters to mail carriers for home delivery, but these carriers received no government salary and their entire compensation depended on what they were paid by the recipients of individual letters. In 1847, the United States Post Office Department adopted the idea of a postage stamp,which of course simplified the payment for postal service but caused grumbling by those who did not like to pi'epay. Besides, the stamp covered only delivery to the post office and did not include carrying it to a private address. In Philadelphia, for example, with a population of 150,000, people still had to go to the post office to get their mail. The confusion and congestion of individual citizens looking for their letters was itself enough to discourage use of the mail. It is no wonder that, during the years of these cumbersome arrangements, private letter-carrying and express businesses developed. Although their activities were only semi-legal, they thrived and actually advertised that between Boston and Philadelphia they Were half-day speedier than the government mail.
The government postal service lost volume to private competition and was not able to handle efficiently even the business it had. Finally, in 1863, Congress provided that the mail carriers who delivered the mail from the post offices to private addresses should receive a government salary,and that there should be no extra charge for that delivery. But this delivery service was at first confined to cities, and free home delivery became a sing of urbanization. In 1890, of the 75 million people in the United States, fewer than 20 million had mail delivered free to their doors. The rest; nearly three quarters of the population, still received no mail unless they went to their post OffiCe.
What is the main idea of the passage? A．The development of a government postal system.
B．The increasing use of private mail services.
C．The history of postal stamps.
D．The comparison of urban and rural postal services.
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