Which of the Following Is TRUE of Gases in the Heterosphere

Multiple Choice

Which of the following is TRUE of gases in the heterosphere?

A) Nitrogen occurs at the top,followed in descending order by helium,oxygen and hydrogen.
B) The nitrogen and oxygen occur at the top of the heterosphere and the hydrogen and helium at the bottom.
C) The hydrogen,helium,oxygen and nitrogen are well mixed.
D) The hydrogen and helium occur at the top of the heterosphere,and the nitrogen and oxygen at the bottom.