The second answer is the correct answer. I know what youre thinking: not on my monitor! But if you were to take a strip of gray paper that was the same shade as A and lay it across A and B, you would be amazed to see that both squares match. Or, you could take the photo into Photoshop and use the eyedropper tool; youll get the same shade of gray whether you click on A or B. Your eyes and brain alone will never be able to tell that they are the same color, though. Our brain has a remarkable (and somewhat scary) ability to interpret and change the way we perceive things. So what does all this mean? We never really see what we think we see because our brain makes stuff up! (And if you dont trust this quiz on this one, go to the researcher, Edward H. Adelsons, website: https://persci.mit.edu/gallery/checkershadow.