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Information and IdeasVery hardInferring scaling, not setup, caused mismatch

Information and Ideas practice question

How air flows around an object depends not on wind speed alone but on a combination of the object's size, the wind's speed, and the air's density and viscosity; a scale model in a wind tunnel therefore does not automatically experience the same flow as the full-size object. Engineers testing a one-fifth-scale model of a bridge deck ran their tunnel at exactly the wind speeds the finished bridge would face and recorded the model's drag. Once the bridge was built, field measurements of its drag differed sharply from that prediction. A later test of the same model in a tunnel pressurized to five times normal air density—which offsets the fivefold reduction in size—matched the field values closely. Together the two tests show that ______

Which choice most logically completes the text with a conclusion the evidence supports?

  1. A. reproducing the full-scale wind speed is not by itself enough to make a scale model encounter the same flow conditions as the full-size structure.Correct
  2. B. wind tunnel tests of scale models cannot be used to predict how full-size structures will behave.
  3. C. the wind speeds used in the first test were set too low to represent the conditions the finished bridge would face.
  4. D. the drag measured on the finished bridge was greater than the drag measured on the model.

Answer: A. reproducing the full-scale wind speed is not by itself enough to make a scale model encounter the same flow conditions as the full-size structure.

The first test matched wind speed but not the full combination of factors, and it failed; the second test restored that combination by raising density and succeeded. The supported conclusion is that matching speed alone is insufficient. B overgeneralizes in a way the second test directly refutes—the pressurized model predicted full-scale drag accurately. C contradicts the text, which states the tunnel ran at exactly the speeds the bridge would face. D invents a direction: "differed sharply" specifies a discrepancy but never says which measurement was larger.

Why the other answers are wrong

B. wind tunnel tests of scale models cannot be used to predict how full-size structures will behave.
The second test refutes this: the model in the pressurized tunnel matched the field values closely, so scale models clearly can predict full-size behavior when the conditions are set correctly.
C. the wind speeds used in the first test were set too low to represent the conditions the finished bridge would face.
The text says the tunnel ran at exactly the wind speeds the finished bridge would face — the speed was right; it was the air density that didn't match.
D. the drag measured on the finished bridge was greater than the drag measured on the model.
The passage says only that the field drag 'differed sharply' from the prediction; it never states which of the two was larger.

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