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Part ACSIR NET December 2025contours-follow-distance-from-the-origin-not-x-or-y-alone

Contours follow distance from the origin not x or y alone

The following plot shows temperature as a function of x and y, brightest at the far corner (x, y) = (5, 5) and darkest at the origin, with the colour banding curving between them rather than running in straight stripes. Along which path is the temperature change minimum?

  1. A.x = constant or y = constant
  2. B.y/x2=y/x^{2} = constant
  3. C.y2+x2=y^{2} + x^{2} = constant✓
  4. D.yx = constant

You have the answer. Trap Analysis is why the other three were written.

Not a worked solution repeated four times — the specific reasoning error each wrong option was built to reward.

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