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Leguminous plants establish symbiotic relationships with soil bacteria known as rhizobia, which convert atmospheric dinitrogen $N_2$ into biologically accessible ammonia. Because the enzymatic reduction of nitrogen requires high cellular energy investments in the form of adenosine triphosphate (ATP), soil phosphorus availability often limits this process. Plant ecologist Dr. Elena Vasquez measured nitrogen fixation rates in soybean plants cultivated under three distinct soil phosphorus concentrations: low ($5mg/kg$), moderate ($20mg/kg$), and high ($50mg/kg$). Vasquez concluded that biological nitrogen fixation scales directly with increasing phosphorus availability.

Which choice best describes data from the graph that support Dr. Vasquez’s conclusion?

A

Soybean plants grown in moderate phosphorus soil exhibited nitrogen fixation rates approximately twice as high as those grown in high phosphorus soil.

B

The nitrogen fixation rate increased from approximately $15\mu \mathrm{mol}{N}_2/\mathrm{plant}/\mathrm{h}$ under low phosphorus conditions to over $60\mu \mathrm{mol}{N}_2/\mathrm{plant}/\mathrm{h}$ under high phosphorus conditions.

C

Plants grown in low phosphorus soil achieved higher nitrogen fixation rates than plants cultivated in moderate phosphorus soil.

D

The nitrogen fixation rate remained essentially constant across all three tested phosphorus concentrations.