Explanation of Answer Choices:

A (Incorrect): The passage does not present "two opposing theories." It describes a natural baseline (monsoonal blooms) followed by a human-influenced escalation. There is no conflict to reconcile; the highlighted sentence bridges historical and modern data toward a unified hypothesis.

B (Incorrect): The sentence does not identify what initially motivated the research (the "impetus" or "catalyst"). The studies are described as investigating sediment cores to understand bloom history, but the highlighted sentence presents a finding from that research, not its original catalyst.

C (Incorrect): The sentence does not undermine or contradict the preceding sentence. The prior sentence establishes that akinetes correlated historically with monsoonal flooding. The highlighted sentence says akinetes surged even more dramatically after 1980 due to fertilizers. This is additive, not contradictory—it layers on a new causal factor rather than refuting the old one.

D (Incorrect): No methodological flaw or problem is identified. The methodology (extracting, dating, counting akinetes) is presented as yielding valid, interpretable results throughout.

E (Correct): The final sentence of the passage states that researchers believe "anthropogenic nutrient loading...has amplified both the magnitude and toxicity of modern blooms far beyond their pre-industrial baseline." The highlighted sentence provides the direct temporal evidence for this claim: the post-1980 surge in akinete abundance coinciding precisely with phosphate fertilizer adoption. It serves as the empirical foundation upon which the concluding hypothesis rests.

Explanation of Answer Choices:

A (Correct — Select): The researchers' hypothesis is that anthropogenic nutrient loading from fertilizers "amplifies" the magnitude and toxicity of blooms beyond what natural processes would produce. From this causal claim, it is a reasonable inference that reducing the anthropogenic input would lessen the amplification—i.e., mitigate severity. Note the wording "would likely be mitigated" is appropriately hedged, not absolute, and aligns with the logic of reducing a causal contributor.

B (Incorrect — Do NOT select): The passage provides no basis to infer that continued fertilizer use would cause akinete concentrations to "plateau and decline." The historical data show a surge after 1980 correlating with fertilizer adoption, implying an increase associated with sustained or growing use. A plateau or decline would require an additional mechanism (e.g., saturation, adaptation) that is neither mentioned nor inferable. The passage's logic suggests continued fertilizer dependence would sustain or exacerbate blooms, not trigger a decline.

C (Incorrect — Do NOT select): The passage explicitly states that algal blooms have been documented "for centuries" and that akinete concentrations were elevated during historical monsoonal flooding prior to the introduction of phosphate fertilizers. This establishes a natural baseline for blooms driven by monsoon-induced phosphorus runoff. Stopping fertilizer use would remove the anthropogenic amplification, but the natural flood-driven blooms would continue. The claim that blooms would "cease to occur" is too absolute and contradicts the passage's historical evidence.