​Why Has Effectively Combating the Spread of Malaria Become More

​Why has effectively combating the spread of malaria become more challenging since the 1970s?

A) ​Anti-malarial drugs cost too much to be used widely in the less-developed countries where malaria infections are widespread.
B) ​During this century,the average atmospheric temperature had risen,thus causing populations of malaria-carrying mosquitoes to spread from tropical areas to warmer temperate areas of the earth.
C) ​The vaccine that can prevent this disease is expensive.
D) ​The parasite infects and destroys its victim's red blood cells,causing intense fever,chills,drenching sweats,severe abdominal pain,vomiting,headaches,and increased susceptibility to other diseases.
E) ​Most species of the Anopheles mosquito have become genetically resistant to most insecticides and the Plasmodium parasites have become genetically resistant to common antimalarial drugs.