Arecent study showed that teenagers who had night light in their rooms as babies are more likely to suffer from myopia than teenagers who had not used night lights. but genetic factors are known to play a role, and near-sighted parents are more likely to install a night light in their children's room. this makes it hard to tell if the night light caused the subsequent myopia. this is an example ofa. double-blindingb. the placebo effectc. confoundingd. controle. an explanatory variable