"After scanning the green labeled areas and the red labeled areas, when we combine the two images, the spots that show up as yellow correspond to "

  ... areas labeled both red and green.

Explanation:

The spots that show up as yellow correspond to areas labeled both red and green.

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The effect of combining images will depend on the method used to create the combination. There are many possibilities when combining color images.

When mixing red and green light, yellow light is produced. When mixing red and green pigment, brown pigment is produced. When mixing digital representations of red and green, the effect depends on the individual representations, the method of mixing, and the interpretation of the mixed result.

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Given the way the question is worded, we assume that yellow is produced by a combination of red and green appearing in the same area of the images. That is, ...

  The spots that show up as yellow correspond to areas labeled both red and green.

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