问题
单项选择题
"An idea" as used in Line 5, refers to ______.
A．a model of matter
B．emission of electromagnetic radiation
C．quantums
D．the equation that described the distribution of radiation accurately over the range of low to high frequencies
答案
参考答案：D
解析：[分析] 细节题型。 从文章第二段的第一和第二句(见上题)中可看出an idea指的即是“描述辐射从低频到高频精确分布的一个方程式”，所以选项D正确。