When Looking at the Carbon Isotopes Present in the Atmosphere

When looking at the carbon isotopes present in the atmosphere, scientists determined that ¹³C and ¹⁴C levels have decreased over time. What did researchers conclude from these observations?

A) volcanic activity is responsible for increased atmospheric CO₂
B) the burning of forests is responsible for increased atmospheric CO₂
C) the use of fossil fuels is responsible for increased atmospheric CO₂
D) increased plant/animal respiration is responsible for increased atmospheric CO₂