Two problems. First, the dumping has been going on for "many years," but the findings are recent. If companies have been dumping waste for years, why would the population declines only show up now? Second, pharmaceutical waste dumped into the ocean wouldn't selectively target P-plankton and three fish species. We'd expect other species in the ecosystem to be affected, too. But the stimulus tells us no other species appear to be affected.

This is the best answer. The bacteria destroys P-plankton cell walls, which accounts for the plankton decline. It attacks the respiratory systems of fish X, Y, and Z, which accounts for their high death rates. And because it's a new strain, this explains why the declines are recent. Most importantly, the bacteria is specific in what it targets: P-plankton and the respiratory systems of fish X, Y, and Z. There's nothing in (B) that suggests the bacteria would harm other species, which is consistent with no other species being affected.

This explains the P-plankton decline (a toxin is killing them), but it says nothing about what's causing the high death rates for fish X, Y, and Z. You might think, "Well, if P-plankton are dying, the fish lose a food source, so they die too." But the stimulus says the fish only sometimes eat P-plankton, and a 10% drop in plankton population doesn't obviously translate to "extraordinarily high death rates" for three fish species. (C) leaves half the mystery unsolved.

This is tempting because it seems to connect the two phenomena: the fish are starving, and the loss of P-plankton is making things worse. But(D) says the fish are experiencing "widespread starvation," and that the loss of P-plankton is driving their death rates "even higher." This means the fish were already dying from starvation before the P-plankton decline made things worse. So (D) doesn't explain why the fish are starving in the first place. It also doesn't explain what caused the P-plankton population to drop.

Same core problem as (A). Climate change affecting the ocean "all along the northeast coast" is a broad, ecosystem-wide force. If global warming changed climatic conditions across the entire coast, we'd expect many species to be affected, not just P-plankton and three fish species. The stimulus is specific that no other species appear to be affected, and (E) gives us no reason to think global warming would be so selective.