Please help

I’m doing some practice exam questions for biology, and i’m not sure how to do these 3 parts to the question.

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

Which are reasons cells undergo cell division?