Use the Following Description to Answer the Question(s) Below

Use the following description to answer the question(s) below.
On the volcanic, equatorial West African island of Sao Tomé, two species of fruit fly exist. Drosophila yakuba inhabits the island's lowlands, and is also found on the African mainland, located about 200 miles away. The very closely related species Drosophila santomea is found at higher elevations and only on Sao Tomé. The two species can hybridize, though male hybrids are sterile. A hybrid zone exists at middle elevations, where hybrids are greatly outnumbered by D. santomea. Studies of the two species' nuclear genomes reveal that D. yakuba on the island is more closely related to mainland D. yakuba than to D. santomea (2n = 4 in both species) . Sao Tomé rose from the Atlantic Ocean about 14 million years ago.
-Which of the following reduces gene flow between the gene pools of the two species on Sao Tomé, despite the existence of hybrids?

A) gametic isolation
B) reduced hybrid viability
C) hybrid sterility
D) temporal isolation
E) a geographic barrier