In the FCC example, which observation indicates that the horizontal and vertical lines are equally dark?

• A. Lines equally dark
• B. Vertical lines darker
• C. Horizontal lines darker
• D. No darkness observed

Answer: (A)

The key idea is comparing how dark the horizontal and vertical lines appear. When they look the same darkness, it means the luminance is balanced across both orientations and there’s no orientation-based bias in perception. That equality is exactly what the FCC example uses to signal that the two sets of lines have matched darkness. If one orientation appeared darker, you’d see a directional bias; if no darkness could be discerned, the comparison wouldn’t be possible. So the observation that the lines are equally dark is the correct indicator.