Refer to the Graph

Refer to the graph. In the FACE experiment in a North Carolina forest, the amount of tree growth (change in carbon biomass per square meter) was measured under conditions of ambient CO₂ (270 ppm in 2000, when measurements were done) and elevated CO₂ (550 ppm) .Half of both the ambient and elevated plots were given additional nitrogen (fertilized) ; the other half were not (unfertilized) .According to the results shown, what can be concluded about the effectiveness of elevated CO₂ on increasing rate of tree growth?

A) Elevated CO₂ increases tree growth in all cases.
B) Elevated CO₂ increases tree growth only when they are fertilized as well.
C) Elevated CO₂ has little effect on tree growth at any time.
D) Elevated CO₂ increases tree growth most when trees are unfertilized.
E) Tree growth is highest under ambient, not elevated, CO₂ levels.

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