Passage Summary: We see color through special sensors in our eyes, and color-blind people lack these sensors. Curiously, even people who aren't color-blind sometimes can't see the difference between certain shades of red that most people notice.

Reasoning: No reasoning (Fact Set).

Analysis: The conflict here is between the general ability to distinguish red from green and the specific inability to distinguish shades of red. Since this is an 'EXCEPT' question, four answers will provide a reason for this discrepancy—perhaps involving variations in the number of pigments, brain processing, or environmental factors. We need to find the one answer that does not explain why someone with basic red-green vision would struggle with specific red nuances. Focus on identifying which choice leaves the mystery of the 10 to 20 percent of subjects completely untouched.