The Following Questions Refer to the Description Below

The following questions refer to the description below.
Malaria is a global health problem in over 100 countries, killing about one million people every year. The protist, Plasmodium, that causes malaria lives in 2 hosts, mosquitoes and humans. Malaria is transmitted to humans by the Anopholes mosquito.
-The use of DDT greatly decreased numbers of Anopholes mosquitoes in the 1960s, and drugs killed the Plasmodium. Now doctors tend to prescribe a combination of drugs against malaria. Why don't doctors prescribe a vaccine to combat malaria?

A) In the presence of multiple drugs, the mutation rate of pathogens decreases.
B) Multiple drugs enhance the immune system better than vaccines.
C) Making an effective vaccine has proved to be difficult because Plasmodium lives inside the host's cells and continually changes its surface proteins.
D) There are fewer side effects associated with using a combination of drugs compared with using a vaccine.
E) Vaccines are more expensive than many drugs.