A drop of pond water observed under a microscope shows moving organisms. Some are single-celled with hair-like flagella, some are chain-shaped green strands. Which two groups are most likely present?

Protozoans (like Amoeba and Paramoecium) are single-celled, often motile, and live in pond water. Green chain-like or filamentous strands are characteristic of algae (e.g. Spirogyra). Viruses cannot be seen under a school microscope, and fungi are not the chain-like green forms described.