Correct Option (D)
The passage details the impact of climate change on the Arctic Ocean, specifically noting earlier and faster sea ice melt, later winter freeze, and a 30% decline in summer ice extent over three decades. It explicitly states that this "lengthening period of summer melt threatens to undermine the whole Arctic food web, atop which stand polar bears." This directly conveys that climate change, through its effects on the Arctic environment and food web, poses a significant threat to the survival of polar bears.
Incorrect Options:
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Option (A): The passage indicates a "lengthening period of summer melt," implying an extended duration of melting conditions, not a shorter Arctic summer. While higher temperatures are an underlying cause of melting, the passage's emphasis on the duration of the melt period contradicts the idea of a shorter summer.
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Option (B): The passage does not propose any solutions or interventions, such as relocating polar bears. Furthermore, the South Pole (Antarctica) represents a distinct ecosystem with different prey species and environmental conditions, making it an unsuitable habitat for polar bears. This option introduces external information not supported by the provided text.
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Option (C): While polar bears are apex predators and crucial to the Arctic food web, their disappearance would lead to significant disruption and alteration of the food chains, potentially causing trophic cascades. However, it is an overstatement to claim that the entire food chain would "disappear." Other trophic levels and species would persist, albeit in a modified ecological balance. The passage does not support such an absolute conclusion.