One-celled microorganisms, some beneficial and some cause disease.

• A. Bacteria
• B. Carnivore
• C. Cell
• D. Food web

Answer: (A)

Single-celled microorganisms include bacteria, which can be helpful—like gut bacteria that aid digestion or bacteria used in fermentation—and harmful when certain strains cause disease. This description fits bacteria because they are the most familiar group of single-celled organisms that can have both beneficial and disease-causing members.

A carnivore is an animal that eats meat, not a single-celled microbe. A cell is the basic unit of life, not a category of organisms. A food web is a network showing who eats whom in an ecosystem, not a description of microorganisms.