Some Scientists Are Trying to Develop a Vaccine That Would

Some scientists are trying to develop a vaccine that would prevent cancer. What is an important consideration?

A) Since cancer cells have self-marking MHC molecules, they cannot be attacked by the immune system.
B) Antigen selection would be critical, due to danger of the body's immune system attacking its own healthy tissue.
C) The natural killer (NK) cells already perform this function, which cannot be enhanced.
D) It is not possible -- vaccines can only protect against viruses or bacteria.