Pileated Woodpeckers (Dryocopus pileatus) excavate large (>45 cm) cavities in trees that they use for nests and roosts. Wood Ducks (Aix spons) also build nests in suitable tree holes, but cannot excavate their own cavities. Consequently, they frequently use nests of Pileated Woodpeckers. Based on this information, which of the following statements is correct? a. Wood Ducks are ecosystem engineers because they nest in cavities built by woodpeckers. b. Pileated Woodpeckers are ecosystem engineers because they excavate tree cavities that Wood Ducks use to build their nests. c. Pileated Woodpeckers are ecosystem engineers because they excavate tree cavities to build their own nests. d. Neither species is an ecosystem engineer by virtue of their nesting habit.