下面是阅读理解原文， 有一道题做错了不明原因，求英语高手解答，不胜感激！
It is only a shiny, yellow metal, but it is one of the most valuable metals on earth. Since the beginning of time, men have looked for this extremely valuable item, gold.
Gold is soft and easy to form into different objects. If it gets wet, it does not rust like iron or other metals. Acids do not change it. One important reason that gold is so valuable is that it is scarce.
Due to its softness, gold must be combined with other metals to harden it and give it strength. By hardening gold, people can then make coins and jewelry with it. Metals commonly mixed with gold are nickel, platinum, and copper. Gold jewelry bears a number and the letter K. The K means karat (carat), which is a measure of the amount of pure gold in the jewelry. Pure gold is 24K. Therefore, a necklace which is 14K is fourteen parts gold and ten parts other metal.
Many countries utilize gold for certain coins, but this practice is not as common now as it once was. The United States, for instance, stopped making gold coins in 1933.
答题：59. If a ring is marked 12K, then the percentage of gold in it is _____.
A. 12 B. 50 C. 25 D. 24
答案给的是B.50k,不知道为什么选B， 50是怎么计算出来的