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ASVAB · Main Idea and Theme

Read: 'The transistor, invented in 1947, replaced bulky vacuum tubes in electronic devices. By reducing size and increasing reliability, it made portable electronics possible. Without it, modern smartphones, computers, and communication technology would not exist.' What is the MAIN IDEA?

Correct answer

The transistor fundamentally enabled modern electronics by replacing vacuum tubes and making miniaturization possible

  1. A Vacuum tubes were unreliable
  2. B The transistor fundamentally enabled modern electronics by replacing vacuum tubes and making miniaturization possible
  3. C Electronics are important
  4. D 1947 was an important year

Why this is the answer

The main idea captures both the specific claim AND its significance. The passage makes two connected points: what the transistor did (replaced vacuum tubes) and why it matters (enabled modern electronics). Option A is too narrow — a single supporting detail. Option C is too broad and vague. Option D identifies only the date. Option B captures the full argument: what it did AND why it matters. Always check that the main idea covers all major sections of the passage.
Source: ASVAB PC, Main Idea

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