Which one of the following best expresses the main idea of the passage? A. There is strong evidence to support the theory that an increase in atmospheric dust has contributed to the severity of the drought in sub-Saharan West Africa. B. The suggestion that Northern Hemisphere cooling is contributing to a decline of rainfall in sub-Saharan West Africa is open to question. C. The expansion of the circumpolar vortex has caused a dramatic shift in the atmospheric circulation patterns above sub-Saharan West Africa. D. The drought in sub-Saharan West Africa represents a long-term permanent alteration in global climate patterns. E. Meteorologists cannot determine when the drought in sub-Saharan West Africa is likely to end.