Well one possible reason could be because it could ruin the land. When land focuses on the mass production of a certain crop, the land loses essential nutrients and will deteriorate overtime.
the answer is all Five you can find in the US.
Mars' atmosphere is made mostly of carbon dioxide, much like Venus. It is not as hot as Venus because the Martian atmosphere is extremely thin. Venus has a thick layer of carbon dioxide gas surrounding the planet. This is why it is the hottest planet in the solar system even though it is not the closest to the sun. Carbon dioxide is a greenhouse gas so it has the ability to trap infrared radiation. The atmosphere on Mars is thin and so it will not trap as much heat as the Venusian atmosphere.
