Percentage Distribution of 6- to 12-year-olds, 1972-97. US Department of Education.
芝加哥风格引文Percentage Distribution of 6- to 12-year-olds, 1972-97. US Department of Education.
MLA引文Percentage Distribution of 6- to 12-year-olds, 1972-97. US Department of Education.
警告：这些引文格式不一定是100％准确.