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

Multiple Choice

Use the following description to answer the question(s) below.
The Isthmus of Panama (a narrow strip of land connecting North and South America) started rising about 10 million years ago. The Pacific and Caribbean oceans were completely separated by the isthmus about 3 million years ago.
Researchers have examined species of snapping shrimp on both sides of the isthmus. Based on the morphological species concept, there appear to be seven pairs of species, with one species of each pair in the Pacific and the other in the Caribbean. The different species pairs live at somewhat different depths in the ocean. Using mitochondrial DNA sequences, the researchers estimated phylogenies and found that each of these species pairs, separated by the isthmus, were indeed each other's closest relatives. The researchers investigated mating in the lab and found that many species pairs seldom courted each other, and those that did mate seldom produced fertile offspring. (N. Knowlton, L. A. Weigt, L. A. Solorzano, D. K. Mills, and E. Bermingham. 1993. Divergence in proteins, mitochondrial DNA, and reproductive incompatibility across the Isthmus of Panama. Science 260:1629-32.)
-There are currently two large, permanent bridges that span the Panama Canal. The bridges are about eight miles apart. If snapping shrimp avoid swimming at night and avoid swimming under shadows, then what do these bridges represent for the snapping shrimp?

A) sources of refuge
B) geographic barriers
C) sources of a hybrid zone between the two bridges
D) sources for increased gene flow