Covers GAAP

Carbon dioxide provides a direct link between the atmosphere and the biosphere because green plants take it from the air during photosynthesis and convert it into organic food. That carbon then moves through food chains and returns to the atmosphere through respiration and decomposition. Argon and helium are largely inert, while metal particles are not a normal biological gas link.

Correct answer: A. The stratosphere contains the major concentration of atmospheric ozone, commonly called the ozone layer. Ozone absorbs much of the Sun’s biologically harmful ultraviolet radiation before that radiation reaches Earth’s surface. This protects living organisms from excessive UV exposure. The same gas has a different effect near the ground: tropospheric ozone is a pollutant that can irritate the respiratory system and damage plants. B is wrong because sea salt is produced mainly by evaporation and other marine processes, not by ozone. C is unrelated; ozone does not drive river flow. D is also unrelated because Earth’s internal heat comes from processes inside the planet, not from ozone. Thus A gives the protective reason, while the location explains why stratospheric ozone is beneficial.