What makes it difficult to control the spread of malaria? 1. International air travel for trade and tourism has risen. 2. The mosquito vector has quickly developed resistance to insecticides. 3. The $\textit{Plasmodium}$ pathogen has very high antigenic variability. 4. Civil unrest and poverty lead to overcrowded living conditions.
- A1, 2 and 3
- B1, 2 and 4
- C2 and 3 only
- D3 only