We can rule out small fish and grasses. My guess is that zooplankton get their energy from phytoplankton, because phytoplankton use photosynthesis to get their energy.
I think the answer would be c. because for birds with longer beaks, it was easier to eat seeds off a cactus (for example), and for birds with shorter seeds, it was easier to eat seeds off the ground.
nutrition is the answer bcz without nutrition the diseases will effect more in a population
