1. A normal cell in a laboratory forms a single layer on a dish. When the cell touches other cells, it halts dividing. Which of the following hypotheses explains this phenomenon? a. It was regulated by kinases and cyclins.

b. Cells grow and divide in response to one growth factor.

c. It happens when internal and external signals stimulate genes that help produce self-destructive enzymes.

d. Receptors on adjacent cells have bound to each other causing the cells’ cytoskeleton to form structures that may hinder the signals that activate the growth.