Question

carbon dioxide levels are increasing because
○ human agriculture has greatly increased the amount of carbon dioxide given off by plants during
respiration.
○ humans have begun releasing carbon that was formerly sequestered in fossil fuels into the
atmosphere.
○ holes in the ozone layer have reduced the total amount of photosynthesis occurring in the biosphere.
○ the huge growth in the human population and domestic livestock has increased the amount of carbon
dioxide being released through respiration.

Explanation:

Plants take in carbon dioxide during photosynthesis and release it during respiration. The amount of carbon dioxide from plant respiration in agriculture is not the main cause of the increase. Holes in the ozone layer mainly affect UV - B radiation and not directly the amount of photosynthesis in a way that would cause a large - scale increase in carbon dioxide. While human and livestock respiration do contribute some carbon dioxide, the major factor is the burning of fossil fuels. Fossil fuels (coal, oil, gas) are formed from ancient organic matter. When burned, the carbon that was stored (sequestered) in them for millions of years is released as carbon dioxide.

Answer:

humans have begun releasing carbon that was formerly sequestered in fossil fuels into the atmosphere.