Here's why:

* Heterotrophs are organisms that cannot produce their own food through photosynthesis. They obtain their energy and nutrients by consuming other organisms or organic matter. This includes:

* Mold: Fungi that grow as multicellular filaments. They decompose organic matter for food.

* Yeast: Unicellular fungi that obtain nutrients from sugars and other organic compounds.

In contrast:

* Autotrophs are organisms that can produce their own food using energy from sunlight (photosynthesis) or from inorganic chemical reactions (chemosynthesis).

So, while mold and yeast are primitive life forms, the key characteristic that defines them is their heterotrophic nature, meaning they cannot make their own food.