Choose the one sentence in this excerpt that best describes how trees play a role in nitrogen-fixing.
A. Some trees were growing out of river gravel, with hardly any access to nitrogen in the soil.
B. University of Washington researcher Sharon Doty got interested in the organisms a couple decades ago.
C. Inside their stems, however, Doty found endophytes that fixed the nitrogen for the trees, no root nodules necessary.
D. She was studying willow and poplar trees, which are among the first trees to grow on disturbed land after events like a volcanic eruption, floods, or rockfall.