Final answer:
The dominant gas in the Earth's atmosphere is nitrogen, and the density of the atmosphere decreases with increasing elevation. Nitrogen comprises approximately 78% of the atmosphere and is crucial for biological processes while the density diminishes with altitude, affecting oxygen availability.
Step-by-step explanation:
The dominant gas in the atmosphere is nitrogen. The density of the atmosphere decreases with increasing elevation. Thus, the correct answer to the question is 'c. nitrogen/decreases'. Nitrogen makes up about 78% of the Earth's atmosphere and is critical for many biological processes, though it is found as nitrogen gas N₂, which is generally unavailable to most living organisms without transformation through the nitrogen cycle. As elevation increases, the concentration of atmospheric gases decreases, which is why mountain climbers often require supplemental oxygen.
The composition of the atmosphere is a significant factor contributing to life on Earth, as it provides essential elements like nitrogen and oxygen for biological processes, including the making of proteins and the facilitation of aerobic respiration. The density of the atmosphere diminishes with altitude due to the exponential decrease in air pressure; this decrease in air pressure and concentration of oxygen at higher altitudes can lead to hypoxia, a condition of insufficient oxygen available to the body, experienced by high-altitude climbers.