Which of the following statements is a correct distinction between autotrophs and heterotrophs?
a. Autotrophs, but not heterotrophs, can nourish themselves beginning with $\mathrm{CO}_{2}$ and other nutrients that are inorganic.
b. Only heterotrophs require chemical compounds from the environment.
c. Cellular respiration is unique to heterotrophs.
d. Only heterotrophs have mitochondria.