Answer:
Most organisms cannot obtain nitrogen from the atmosphere. Nitrogen fixing bacteria take Nitrogen out of the atmosphere and make it available for consumption by the other organisms, This is important because Nitrogen is an essential building block of life.
The answer would be 3...mass!
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A cold front does count as weather, if you're looking for more info on cold fronts and warm fronts visit the site below.
http://www.weatherquestions.com/What_are_fronts.htm