A. S phase immediately precedes cell division
Cytosine (C) and thymine (T) are the smaller pyrimidines. RNA also contains four different bases. Three of these are the same as in DNA: adenine, guanine, and cytosine. RNA contains uracil (U) instead of thymine (T).
Here's link to the answer:
tinyurl.com/wpazsebu
My answer would be radiation.
I think its when there is an over growth of cells.