According to the passage proponents of the cooling hypothesis suggested that the circumpolar vortex is likely to expand when which one of the following occurs? A. The average annual atmospheric temperature of the tropics is significantly higher than normal for an extended period of time. B. The average annual snowfall in the Northern Hemisphere is lower than normal for an extended period of time. C. The average annual surface temperature of Northern Hemisphere waters is higher than the average annual surface temperature of Northern Hemisphere landmasses. D. There is a significant increase in the difference between the average annual atmospheric temperature of the tropics and that of the more northern latitudes. E. There is a significant increase in the difference between the average annual atmospheric temperatures of the middle and the high latitudes in the Northern Hemisphere.