SAT Question cbdd5287: Command Of Evidence answer and explanation

Choice C is the best answer because it most logically completes the text based on supporting data in the graph. The text indicates that in the fly D. melanogaster, DptA and DptB are genes that encode peptides that both fight pathogens and promote beneficial microbes. Researchers tested D. melanogaster's resistance to P. rettgeri and A. sicerae bacteria based on which variation of the peptide-encoding gene the flies exhibit: DptA silenced (referred to as type A), DptB silenced (type B), or both silenced (type AB). The text also indicates that resistance to P. rettgeri correlates with DptA activity but not with DptB activity (which would manifest as type B flies surviving at a higher rate than other fly types when exposed to P. rettgeri). The graph shows the post–A. sicerae infection results, which indicate that DptB activity was most strongly associated with survival, whereas DptA activity was not (manifesting in the graph as the type A flies having greater survival rates than the other fly types). In other words, when DptA activity was silenced, the flies showed relatively high survival rates, but when DptB activity was silenced, whether on its own or in conjunction with DptA activity being silenced, survival rates were low, suggesting that DptB may have developed as a specific defense against A. sicerae.