Living things that cannot make there own food are called what

Ineed to know why d is the correct answer asap

Which of the following describes a boom period? a. as one population increases, another population decreases. b. as one population increases, the other population also increases. c. as one population decreases, another population increases. d. as one population decreases, another population also decreases

Give an example of a trait that is controlled by more than one gene.