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Question

Difficulty: MediumDetermining Paragraph-Level Main Ideas

The following passage is adapted from an article about deep-sea marine biology.

[Paragraph 1]
Deep-sea bioluminescence is often associated with mate attraction or active predator deterrence, but its application in camouflage is perhaps its most remarkable role. In the mesopelagic zone, where faint sunlight still penetrates from the surface, organisms are highly vulnerable to predators looking upward from the darkness.

[Paragraph 2] (lines 11–24)
To combat this vulnerability, many species utilize a strategy known as counterillumination. These organisms possess light-emitting organs called photophores, primarily located on their ventral, or underside, surfaces. By producing light that precisely matches the color, wavelength, and intensity of the downwelling sunlight, the organism effectively erases its own silhouette. A predator swimming below cannot distinguish the illuminated underside of the prey from the ambient sunlight filtering down. Consequently, by dynamically adjusting their light output to match shifting surface conditions, these organisms achieve near-invisibility.

[Paragraph 3]
Unlike passive camouflage, which relies on static coloration, counterillumination requires active sensory feedback and metabolic energy. While costly, this dynamic adaptation is crucial for survival in an environment devoid of physical hiding places, demonstrating the complex evolutionary pressures of the open ocean.

Which of the following best states the main idea of Paragraph 2 (lines 11–24)?

  1. It describes the physical mechanism by which deep-sea organisms use ventral photophores to match downwelling light and mask their silhouettes from predators below.Answer
  2. B
    It argues that bioluminescence is a costly but essential survival mechanism that serves various evolutionary functions in the mesopelagic zone.
  3. C
    It points out that photophores are specialized light-emitting organs found on the ventral surfaces of deep-sea creatures.
  4. D
    It asserts that predators looking upward from the deep ocean can easily distinguish ventral light emission from natural downwelling sunlight.

Answer

The correct answer describes the physical mechanism of counterillumination, explaining how ventral photophores match downwelling light to prevent predators below from seeing the organism's silhouette.
The correct answer accurately captures the primary purpose of the paragraph, which is to explain the physical process of counterillumination, detailing how underside photophores emit matching light to hide silhouettes from predators below.

Step-by-Step Solution

1
Analyze the target paragraph (Paragraph 2) to identify its primary focus.
The paragraph explains counterillumination: how photophores on the underside of a marine organism emit light to match the intensity and color of sunlight, erasing its silhouette from predators looking up.
This establishes the core point of the specific paragraph, separating it from surrounding paragraphs.
2
Evaluate the answer choices against the identified focus of the paragraph.
The correct answer accurately summarizes this mechanism, while wrong options either cover the whole passage, highlight a minor detail, or contradict the text.
This ensures the selected option represents the main idea rather than a detail, a contradiction, or a passage-level theme.

Key Concept

Identifying the primary focus of a single paragraph by separating its core argument from minor details and global passage themes.
Estimated Time:1m 30s
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Topics

Determining Paragraph-Level Main Ideas

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