Pick from: 1. passenger pigeons; 2. parasites; 3. sunlight; 4. animal plankton; 5. ice; 6. horses; 7. absorb light energy; 8. honeybees;

Fact 1: Zooplankton are tiny animals that feed on phytoplankton. Fact 2: Planktonic plants are called phytoplankton. Given the two facts above, what feed on plant plankton?
Answer:
4.