After Reading the Paragraphs Below, Answer the Questions That Follow

Multiple Choice

After reading the paragraphs below, answer the questions that follow.
Exposure to the HIV virus doesn't necessarily mean that a person will develop AIDS. Some people have genetic resistance to infection by HIV. Dr. Stephen O'Brien from the U.S. National Cancer Institute has recently identified a mutant form of a gene, called CCR5, that can protect against HIV infection. The mutation probably originated in Europe among survivors of the bubonic plague. The mutated gene prevents the plague bacteria from attaching to cell membranes and, therefore, from entering and infecting body cells.
Although the HIV virus is very different from the bacteria that causes the plague, both diseases affect the exact same cells and use the same method of infection. The presence of the mutated gene in descendants of plague survivors helps prevent them from contracting AIDS. Pharmaceutical companies are using this information as the basis for a new approach to AIDS prevention. This could be very important in areas of the world where the mutation is scarce or absent, such as Africa.
-Imagine that a pharmaceutical company was successful at producing a drug based on the CCR5 gene product that is effective at preventing the contraction of AIDS. However, shortly after the drug has been in use, patients and doctors report that the drug is not as effective as it once was. What is the most likely explanation for this result?

A) The people taking the drug have built up a tolerance to the drug.
B) Some HIV viruses have genetic variations in the RNA genome that provide resistance to the actions of the drug.
C) The HIV virus gained mutations in its DNA genome in order to become resistant to the actions of the drug.
D) The DNA of white blood cells of the people taking the drug have mutated to become resistant to the drug.