题目内容：
根据下面内容，回答题： Gold used in jewelry is mixed with harder metals to add strength and durability. The metals added can also be used to change gold' s color, giving it a fashionable rose or white tint, or to lighten or darken the natural yellow, tone of pure gold.Mixtures like these, of less costly metals with more valuable ones, are called alloys. Copper and silver are the most common metals mixed with gold to make yellow gold jewelry. White gold is usually made with an alloy of gold and nickel.
The measure of gold' s purity is called a karat. The higher the karat rating,the higher the amount of pure gold: 24 karat is pure gold,18 karat is 75% pure gold,14 karat is 58.5% pure gold, and 9 karat is 37.5% pure gold. All other things being equal ,the higher the percentage of pure gold used in the alloy,the more valuable and expensive the jewelry wll be.
Gold jewelry pieces are usually stamped with a marking to identify the karat amount. While gold that is 24K is too soft for jewelry,18K,14K and 9K gold are all appropriate for jewelry, and they all make pieces that look great and wear beautifully.
Which of the following statements best.captures the main idea of this passage? A．Although gold is very valuable,it is also very expensive.
B．Gold jewelry is stamped with its karat ,weight.
C．Gold jewelry is made using alloys.
D．Colored gold is more valuable than white gold.
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