A、Some precious metals. B、Silver and gold. C、Various expensive metals. D、Some inexpensive metals. D录音中提到银币时代已经过去，今天的硬币中已经不含银和其他

Less than 40 years ago in the United States. it was common to change a one-dollar bill for a dollar’s worth of silver. That is because the coins were actually made of silver. (26)But those days are gone. There are no silver or other precious metals in today’s coins. When the price of the precious metal rises above its face value as money, the metal will become more valuable in other uses. Silver coins are no longer in circulation because the silver in coins is worth much more than their face value. A silver firm could find that it is cheaper to obtain silver by melting down coins than by buying it on the commodity markets.
    Gresham’s Law, named after Sir Thomas Gresham, argues that "good money" is driven out of circulation by "bad money". (27)Good money differs from bad money because it has higher commodity value.
    Gresham lived in the 16th century in England where it was common for the value of coins to be decreased. Governments did this by mixing cheaper metals with gold and silver. (28)The governments could thus make a profit in coinage by issuing coins that had less precious metal than the face value indicated. Because different mixings of coins had different amounts of gold and silver, even though they bore the same face value, some coins were worth more than others as commodities. Gresham observed that coins with a higher content of gold and silver were kept rather than being used in exchange, or were melted down for their precious metal. In the mid-1960s when the U. S. issued new coins to replace silver coins, Gresham’s law went right in action.
26. Q: What are today’s coins in the United States made of ?
27. Q: What is the difference between "good money" and "bad money"?
28. Q: Why did the governments issue new coins by mixing cheaper metals in the 16th century?