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Difficulty: HardAuthor's Primary Purpose

Ocean temperature layers significantly distort underwater acoustic signals, creating "shadow zones" where conventional active sonar cannot penetrate. To counter this acoustic refraction, naval engineers developed variable-depth sonar (VDS) systems. By lowering a transducer below the thermocline—the boundary layer where temperature drops abruptly—operators can bypass surface thermal traps and detect submerged vessels hiding in deeper water. Although deploying VDS increases hydrodynamic drag and demands sophisticated towing winches, the tactical advantage of penetrating refractive acoustic barriers makes it an indispensable asset in modern anti-submarine warfare.

What is the author's primary purpose in writing this passage?

  1. A
    to outline the comprehensive evolution of modern anti-submarine warfare tactics
  2. B
    to argue that active sonar systems should be decommissioned due to thermal distortion
  3. to explain how variable-depth sonar overcomes thermal barriers to detect submerged vesselsAnswer
  4. D
    to highlight the mechanical hazards and hydrodynamic drag associated with towing winches

Answer

to explain how variable-depth sonar overcomes thermal barriers to detect submerged vessels
The correct answer accurately identifies the author's main objective: providing an informative explanation of how variable-depth sonar bypasses thermocline distortion to find submerged targets.

Step-by-Step Solution

1
Analyze the structural progression and subject matter of the text.
The text establishes an acoustic challenge (temperature distortion and shadow zones) and details how an engineering innovation (variable-depth sonar) addresses it.
Determining the primary purpose requires identifying the central problem-and-solution structure that unifies the passage.
2
Evaluate the rhetorical tone and communicative goal.
The tone is explanatory and informative, centered on the mechanism of bypassing the thermocline.
Distinguishing between explanatory, persuasive, and purely descriptive aims ensures the selected purpose matches the author's intent.
3
Differentiate between the overarching purpose and secondary supporting details.
The drawbacks regarding drag and winches are subordinate details, while the core message is the functional capability of the system.
High-level distractors frequently spotlight minor factual details or overextend the scope beyond what the author intended.

Key Concept

Author's Primary Purpose
Estimated Time:1m 15s
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Author's Primary Purpose

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