题目内容：
Which of the following statements best explains the relationship between gold and its karat rating? A．The lower the karat rating,the purer the gold．
B．The higher the karat rating,the more expensive the gold．
C．The higher the karat rating,the less valuable the mixture．
D．The lower the karat rating,the purer the alloy.
参考答案：
答案解析：