United Nations Population Fund. (1999). The State of world population 1999: 6billion : a time for choices. UNFPA.
芝加哥风格引文United Nations Population Fund. The State of World Population 1999: 6billion : A Time for Choices. New York: UNFPA, 1999.
MLA引文United Nations Population Fund. The State of World Population 1999: 6billion : A Time for Choices. UNFPA, 1999.
警告：这些引文格式不一定是100％准确.