Researchers Have Shown That "Income Levels in Countries Hard Hit

Researchers have shown that "income levels in countries hard hit by malaria are 33% lower than in countries with similar income that are free of the disease." Which of the following statements is correct regarding this observation?

A) This observation suggests that symptoms from malaria cause patients to lose pay because they miss work.
B) This observation is an example of a correlation and therefore causality cannot be inferred as confidently as if we have results from a manipulated experiment.
C) In order to understand the cause of this observation, scientists should eradicate malaria in ten countries but not in ten other comparable countries in order to experimentally test whether malaria caused the low income levels.
D) The observation demonstrates that correlations provide excellent indications of causality.