A recent article claims that “Green Spaces Make Kids Smarter." The study described in the article involved 2,623 school children in Barcelona. The researchers measured the amount of greenery around the children's schools, and then measured the children's working memories and attention spans. The children who had more vegetation around their schools did better on the memory and attention tests. A: suggest a possible confounding variable, and explain why it meets the requirements of a confounding variable. ​