Which relationship below is an example of competition in an ecosystem?
a. birds eat the insects that cattle disturb while grazing, and they occasionally ride on the cows' backs.
b. bald eagles and black bears fish from the same mountain lake.
c. squirrels nest in an oak tree, and insects live under the ground near the tree.
d. rabbits eat the grasses and shrubs in a meadow, and wolves eat the rabbits in the meadow.