A. The primary corona is not working.
B. The transfer corona is not working.
C. The fuser is not working.
D. The photosensitive drum has been scarred by a staple or paperclip.
The correct answer is B.
The job of the transfer corona is to create a strong static charge on the paper with the opposite polarity of the charge held by the toner, creating a strong attraction between the two. When the transfer corona fails, the paper carries no static charge to speak of, resulting in the paper still being more attractive to the toner than the drum that carries the toner, but not attractive enough to force enough toner to leave the drum to create a sufficiently dark image.
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