A、To make coins lighter. B、To conserve silver. C、To use up surplus copper. D、To make coins last longer. D细节推断题 文中谈到用合金铸造硬币是为了确保它

A silver dollar is not worth a dollar--nor is any other silver coin really worth its face value. Silver coins are made of an alloy--a mixture of 10 per cent copper and 90 per cent silver---which lasts much longer in circulation than pure silver would. A silver dollar contains 90 cents worth of silver and a dime has 9 cents worth. It takes only a pennyworth of silver to make a nickel.
    This was not the case in 1792, when the United States began minting coins of pure gold and silver. Citizens were welcome to stop in anytime with gold or silver in any form--cups, vases, or even gold fillings from their teeth--and have it melted down for spending money.
    Today the mints in Philadelphia and Denver earn a yearly profit of about ＄40 million. Most of this comes from using alloys rather than pure silver in the production of the ＄2.5 billion worth of coins circulating in the United States. The Philadelphia plant also manufactures coins for foreign countries. In a recent year almost 130 million coins were made for the use of foreign governments.