Four tropical zone cities in the Southern Hemisphere.

Plants can have a positive and negative effect on the rate of weathering

Read the article titled "trans-saharan migration routes to north africa and the eu: historical roots and current trends." why do tens of thousands of sub-saharan africans try to cross the mediterranean each year? how are these refugees commonly portrayed? (site 1)

Given: circle a externally tangent to circle b. how many possible common tangents to circles a and b can exist? 2 3 4