【正确答案】 D

【答案解析】由题干关键词mental ratio可定位到第四段倒数第二句话Soon，however，the “mental ratio” was introduced；that is to say，the ratio of the mental age to the chronological age．说明智力比商等于心理年龄(mental age)与实际年龄(chronological age)的比率。因此选[D]。