There have long been dead zones—water too low in oxygen to sustain most forms of life—in the Gulf of Mexico, which receives the waters of the Mississippi River. Scientists studied sediment cores from areas where the gulf’s most recent dead zone occurred. The scientists dated the sediment and counted species of foraminifera (marine protozoans) in the sediment; these species thrive in low-oxygen waters. As far back as 1823, the foraminifera thrived especially during Mississippi River flood years (during which nutrients levels increase), suggesting that nutrients in floodwaters can trigger low-oxygen water. The foraminifera in the core samples were most abundant after 1950, when farmers began using some fertilizer, which is rich in nutrients. Researchers believe that increased use of fertilizer leads to more-extreme dead zones.

  1. Which of the following best describes the function of the highlighted sentence in the context of the passage as a whole?

  A. It reconciles two opposing theories that were discussed earlier in the passage.

  B. It suggests what the initial impetus was for the studies mentioned earlier in the passage.

  C. It undermines a hypothesis that was presented in the preceding sentence.

  D. It describes a problem with the methodology of the studies discussed in the passage.

  E. It provides evidence supporting the belief mentioned in the final sentence of the passage.

  2. It can be inferred from the passage that the “researchers” would agree with which of the following statements about the use of fertilizer? (Consider each of the choices separately and select all that apply.)

  A. If farmers near the Mississippi River decreased the amount of fertilizer that they used, the severity of the dead zones could be diminished.

  B. If farmers near the Mississippi River continue their dependence upon fertilizers, foraminifera population will eventually decline.

  C. If farmers near the Mississippi River stopped using fertilizers altogether, there would no longer be dead zones.




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