(will ) knowing that greenhouse gases are contributing to climate change, how can we reduce our carbon footprint, particularly in the united states? could hydrogen fuel replace fossil fuels in the future? why or why not? should the size of a person’s carbon footprint be limited by law? why or why not? ( give an educated answer)

You can reduce our carbon footprints by using renewable energy,calculate your carbon footprint and make changes,use car less and minimize driving and walk or cycle more and reduce water use.
Hydrogen fuel can not replace fossil fuels this is because fossil fuel use more energy and make more pollution.
 

As explained below.

Explanation:

It's true that greenhouse gases have been on the planet for centuries and that the united states along with the other counties of the world are contributing towards this massive climate change event that happens due to the release of the harmful gases and other waste chemicals. Many new technologies have been developed by the united states government to overcome the impacts of carbon dioxide like the carbon credits and sanctions imposed on nations, and mining of air for carbon as a source of natural energy but still lots of it's on paper and not completely implemented. Hydrogen fuel is the non-conventional type of fuel that is better off than the carbon-producing fossil fuel but the appropriate technology is not yet properly distributed in each country the US can take advantage of this. The human footprint should be at least limited by the law as its necessary for every nation to preserve and protect its natural resource base.

Most smaller stars don't have a radiative zone.