Question

some protists require external input of nutrients and are referred to as ______.

autotrophs

biotrophs

heterotrophs

mixotrophs

Explanation:

1. Autotrophs produce their own nutrients via processes like photosynthesis, so they do not need external nutrient inputs.
2. Biotrophs are parasites that only feed on living host tissue, this is a narrower subset of heterotrophy not matching the general definition.
3. Heterotrophs cannot synthesize their own organic nutrients, so they must obtain them from external sources, matching the question description.
4. Mixotrophs can use both autotrophic and heterotrophic nutrient acquisition strategies.

Answer:

C. Heterotrophs