Answer:
A. Their numbers of atoms
Explanation:
It’s the second one if you need more help tell me
Air is a mixture of gases (and some dust) because the gases (oxygen, nitrogen, carbon dioxide, etc.) do not combine chemically into other compounds. Just like a mixture of salt & pepper doesn't combine into some other thing.
The mixture is homogenous if it consists only of gases. If dust is mixed in the mixture is heterogeneous (dust is solid matter, gas is gas) A heterogenous mixture is one of different types of matter.
Separating one gas from the other is different for each gas and a complicated matter.
The smaller the surface area, the faster the rate of reaction will be. Think of ice. A full sized ice cube will take longer to melt than crushed ice. Crushed ice will melt much quicker because there is more surface area exposed. So if the surface area can be increased, so will the rate of reaction.
Hope this helps! :)