The greenhouse effect is the trapping of a stars warmth in the lower atmosphere of a planet by gasses in the air trapping the energy. This is similar to what actual greenhouses do, where they trap the warmth and humidity of the air inside it so plants can flourish. Farm animals produce a huge amount of the greenhouse gasses required for our planet to trap the suns warmth.
When a scientific experiment is carried out in a controlled setting, all variables are kept the same except for the C. experimental. Experimental variables are the things you are studying in the experiment and it is the only variable you can change.
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dear user nitrogen fixation is so important bcoz of the reasons given above