Explanation
Correct answer: C.
Choice C is the best answer because it most effectively uses data from the graph to exemplify the researcher's claim—namely, that the duration of ice on some lakes has declined between the early 1980s and the mid-2000s. According to the graph, Oulujärvi had surface ice for nearly 200 days in the winter of 1980–81 but only about 160 days of ice in the more recent winter of 2005–06, evidence of a clear decline in the duration of surface ice between these time periods.
Why the other choices are wrong
Choice A
Choice A is incorrect because it focuses on only one period of time (the early 1980s). Thus, the data it cites don't support the researcher's claim, which compares the duration of surface ice across two specific periods of time (the early 1980s and the mid-2000s).
Choice B
Choice B is incorrect because although it accurately describes data from the graph, it contradicts the researcher's claim about declining surface ice duration. It describes an increase, not a decline, in the duration of surface ice on Lake Neusiedl between the early 1980s and the mid-2000s (from about 80 days in 1980–81 to about 100 days in 2005–06). Therefore, the example of Lake Neusiedl wouldn't logically support the claim that some lakes have seen a decline in the duration of surface ice.
Choice D
Choice D is incorrect because it provides information that doesn't support the researcher's claim about declining ice duration. The graph could indicate that both Lake Neusiedl and Oulujärvi had more than 105 days of ice in the winter of 2005–06, but on its own, this information doesn't demonstrate a decline in ice duration between the early 1980s and the mid-2000s.