Most multicellular organisms are composed of cells with different shapes and sizes, and those cells perform different functions. what is the best explanation for this? a. mutations accumulate in an organism's cells, causing them to become different from the other cells. b. the structure and function of an organism's cells is determined by the genetic material it receives during fertilization. c. as cells divide, they only pass on the genetic material that will be used in the daughter cells. d. environmental conditions influence gene expression, which determines how a cell develops.