Which Statement Best Explains Contact Inhibition

Which statement best explains contact inhibition?

A) Contact between neighbouring cells triggers reactions that lead to inhibition of mitosis.
B) As cell number increases, the level of waste products increases, consequently slowing metabolism and leading to mitosis.
C) As cell number increases, the protein kinases they produce compete with neighbouring cells, thus inhibiting mitosis.
D) As neighbouring cells become more tightly packed together, their size is restricted and cytokinesis can no longer occur.