Which of the following is the best example of how diseases have been powerful selective agents for ...

Which of the following is the best example of how diseases have been powerful selective agents for humans, particularly before the arrival of modern medicine?
 
  A. Smallpox, which appeared after people and animals started living together, has worked as a selective agent against people with sickle-cell anemia.
  B. Blood type A individuals are more prone to stomach and cervical cancer. Since these diseases usually occur after reproduction has ended, they are particularly powerful agents in adaptation and evolution through natural selection.
  C. Blood type O will soon become something of the past, since it does not confer an advantage to any disease.
  D. Smallpox, which appeared after people and animals started living together, has worked as a selective agent for people with blood types B and O who have an ability to produce antibodies against smallpox.
  E. Diseases no longer work as powerful selective agents for humans, thanks to the widespread availability of modern medicine.