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Expression of IdeasMediumExplaining how something works

Expression of Ideas practice question

While researching a topic, a student has taken the following notes: - A vacuum flask has two walls with a vacuum between them. - Heat cannot cross a vacuum by conduction or convection. - The inner surfaces are silvered. - Silvered surfaces reflect radiant heat back.

The student wants to explain how a vacuum flask keeps a drink hot. Which choice most effectively uses relevant information from the notes to accomplish this goal?

  1. A. The flask's two walls enclose a vacuum, which heat cannot cross by conduction or convection, and their silvered surfaces reflect radiant heat back into the drink.Correct
  2. B. A vacuum flask keeps a drink hot for many hours.
  3. C. A vacuum flask has two walls.
  4. D. A vacuum flask contains a small heating element that keeps the drink warm.

Answer: A. The flask's two walls enclose a vacuum, which heat cannot cross by conduction or convection, and their silvered surfaces reflect radiant heat back into the drink.

The goal is HOW. Choice A gives both mechanisms from the notes: the vacuum and the silvering.

Why the other answers are wrong

B. A vacuum flask keeps a drink hot for many hours.
This says that the flask works, not how.
C. A vacuum flask has two walls.
Two walls alone don't explain anything.
D. A vacuum flask contains a small heating element that keeps the drink warm.
The notes mention no heating element.

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