Final answer:
Algae are autotrophic protists that can be found in various environments, including freshwater and marine habitats. They can be both unicellular and multicellular. They are responsible for the production of approximately 70% of the oxygen and organic matter in aquatic environments.
Step-by-step explanation:
Algae are autotrophic protists that can be found in various environments, including freshwater and marine habitats. They can be both unicellular and multicellular. Algae are photosynthetic organisms that extract energy from the sun and release oxygen and carbohydrates into their environment. They are important ecologically and environmentally because they are responsible for the production of approximately 70% of the oxygen and organic matter in aquatic environments. Additionally, some types of algae are regularly eaten by humans and other animals. Therefore, statements C) Algae can be found in various environments, including freshwater and marine habitats and D) Algae are always multicellular organisms are true about algae.