Two species of buttercup can be found in the same field in South Dakota. They emerge at different times: One species emerges and flowers in early spring, and the other species emerges in late summer, after the first species has set seed. Both flower species are pollinated by the same insect species, which is common in spring and summer. The different flowering times are likely due to

a. competitive exclusion. b. niche fragmentation. c. niche realignment. d. resource partitioning. e. parasitism.