Answer:
B!
Step-by-step explanation:
Cancer is a essentially a mistake in a cell's DNA. This is very gradual, and takes a while. The immune system deals with most of them, but some slip out of reach and continue to mutate until they grow into tumors, or large pockets of cancer cells. Since their genetic code is very messed up at this point they have no limitations on growth.
P.S. This is my first biology answer! Did I explain it good enough?