Why Does Chemotherapy Work Best for Early Stages of Cancer

Why does chemotherapy work best for early stages of cancer instead of late stages?

A) Chemotherapy works best on cells that are dividing rapidly, such as those in early stage tumors.
B) Chemotherapy works better on a smaller number of cells because cells that are not destroyed can be cleaned up by the immune system.
C) Chemotherapy is more effective the longer the chemical treatment.
D) all of the above
E) none of the above