Due to the widespread resistance of the malaria parasite to drugs such as chloroquine, efforts are being made to develop malaria vaccines to fight malaria. What is the difficulty in developing an effective malaria vaccine?

• A. The causative agents of malaria are different species of Plasmodium.
• B. Humans do not develop immunity to malaria during natural infection.
• C. Vaccines can only be developed against bacteria
• D. Man is the only intermediary host, no other host