Trout and Amoebas Are Both Heterotrophs, and Can Inhabit Similar

Trout and amoebas are both heterotrophs, and can inhabit similar environments (for example, a freshwater pond) . However, these organisms are very different when it comes to their shape and size. Why?

A) Trout and amoebas utilize different sources of CO₂.
B) Trout and amoebas utilize different sources of C₆H₁₂O₆ (or other organic compounds) .
C) Trout and amoebas utilize different sources of O₂.
D) Trout and amoebas have different metabolic pathways.
E) Trout and amoebas are exposed to different environmental conditions, even when they inhabit the same pond.