It Was Discovered That a Larger Proportion of Children Who

It was discovered that a larger proportion of children who slept with nightlights later developed
Nearsightedness, compared to children who did not sleep with nightlights. The headlines read,
"Leaving a light on for you children causes nearsightedness!" Later it was pointed out that
Nearsighted people have more trouble seeing in the dark and are more likely to leave lights on at
Night for their kids. And those same nearsighted parents are likely to have nearsighted kids. This is
An example of

A) a control group.
B) a randomized block design.
C) a lurking variable.
D) a placebo.
E) bias.