21. Dictyostelium discoideum is an unusual organism, one that straddles the boundary between the unicellular and the multicellular. Its feeding phase consists of individual amoeba-like cells that move independently, feeding on bacteria by phagocytosis. When the food runs out, cells begin to aggregate into a multicelled structure that migrates toward light. The cells differentiate into a base, stalk, and spores; only the spores survive to colonize a new habitat. Dictyostelium discoideum is which of the following?
a. An autotroph
b. A heterotroph and autotroph
c. A heterotroph
d. A detritovore
e. A heterotroph and detritovore