Which of the following sets of gases would be most difficult to separate?

Which of the following sets of gases would be most difficult to separate if the method of gaseous effusion is used?
N2 and H2
O2 and CO2
H2 and CH4
He and Ne
O2 and He

Comments on "Which of the following sets of gases would be most difficult to separate?"
More: Does water increase the pH level of lemon juice? Grandma has p[hone 5? So our teacher gave us these practice questions for review. I am completely unsure on how to do it. Can anyone help 10 points!? Can my baby turn out white or light skin? Maths Probability question for maths genius to solve?