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Difficulty: HardExplicit Main Idea

In modern military and commercial aviation, fuel tank ullage—the empty volume above liquid fuel—contains flammable hydrocarbon vapors that pose severe explosion hazards if exposed to electrical arcing or static discharge. While mechanical flame arrestors and reinforced bladders mitigate certain localized risks, onboard inert gas generation systems (OBIGGS) remain the primary defense against fuel tank ignition by continuously displacing oxygen with nitrogen-enriched air. These systems utilize permeable hollow-fiber membranes to separate and vent oxygen molecules from pressurized bleed air, driving ullage oxygen concentrations below nine percent by volume. Consequently, establishing an inert atmosphere through active nitrogen displacement prevents catastrophic vapor detonation under varying flight altitudes and operational temperatures.

Which of the following statements most accurately expresses the main idea explicitly stated in the passage?

  1. A
    Permeable hollow-fiber separation membranes strip oxygen molecules from bleed air to maintain tank oxygen levels below nine percent.
  2. B
    Aviation safety mandates eliminating all flammable hydrocarbon vapors across every internal aircraft compartment.
  3. Onboard inert gas generation systems serve as the principal protection against fuel tank ignition by displacing oxygen with nitrogen.Answer
  4. D
    Mechanical flame arrestors and reinforced bladders provide the primary line of defense against fuel vapor detonations.

Answer

Onboard inert gas generation systems serve as the principal protection against fuel tank ignition by displacing oxygen with nitrogen.
The passage explicitly states that while other methods address localized hazards, onboard inert gas generation systems (OBIGGS) remain the primary defense against fuel tank ignition by displacing oxygen with nitrogen-enriched air. The correct option directly paraphrases this central claim.

Step-by-Step Solution

1
Analyze the structural organization of the passage to locate the central topic assertion.
Identified the explicit thesis stated in the second sentence: onboard inert gas generation systems (OBIGGS) are the primary defense against fuel tank ignition by displacing oxygen with nitrogen-enriched air.
An explicit main idea is directly articulated within an identifiable topic sentence rather than implied.
2
Differentiate the overarching claim from subordinate technical details and secondary countermeasure mentions.
Membrane mechanics, oxygen percentage thresholds, and mechanical arrestors serve solely as supporting evidence illustrating how and why nitrogen displacement functions as the primary protective measure.
Supporting details elaborate on the mechanism but do not encapsulate the complete central purpose of the text.
3
Evaluate the options to find the exact conceptual match that avoids scope distortions and detail traps.
The statement identifying OBIGGS as the principal defense against fuel tank ignition via nitrogen displacement precisely matches the topic sentence.
The correct option must encompass the main idea without narrowing to a single detail or generalizing beyond the text.

Key Concept

Explicit Main Idea Identification
Estimated Time:1m 15s
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Explicit Main Idea

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