题目

铜元素有质量数为63和65的两种天然同位素，它的近似相对原子质量是63.5，则同位素63Cu所占的质量百分比为（ ） A . 20% B . 25% C . 74.4% D . 75% 答案：CWhen first ________ to the market, these products enjoyed great success. [　　] A．introducingB．introduced C．introduceD．being introduced