What was the main difference between the experimental and control study plots in the Free-Air CO2 Enrichment (FACE) experiment concerning the effects of atmospheric gases on plant growth?
a) The experimental plots were exposed to elevated CO2 levels, while the control plots were not
b) The experimental plots were exposed to elevated O2 levels, while the control plots were not
c) The experimental plots were exposed to reduced CO2 levels, while the control plots were not
d) The experimental plots were exposed to reduced O2 levels, while the control plots were not