Why can't krypton and xenon separate during the separation of gases in the air?

The remaining gas is a mixture of argon, neon, krypton, and xenon. Because these elements are chemically very unreactive, chemical means cannot be used to separate them.

Explanation:

They're in Group 0 as they have full outer shells meaning they are chemically unreactive as they already have full outer shells meaning they don't want to lose or gain any electrons. Thus, they can't separate during the separation of gases in the air.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.

i do not get it   at all