A study of kindergarten students has shown that children from households that own many books, an average of 54 books, show average or high reading achievement levels, while the households of children with lower reading achievement levels tend to own fewer books, 5 books on average. What additional information would strengthen the assertion that parents who don't buy children's books to keep in the home cause their children to fall behind their peers in reading skills? ['Children whose parents buy them books feel happier about their lives.', 'The non-school hours of every child in the study were entirely filled with sports activities, television watching, and playing with friends.', 'When children have books of their own, they develop more advanced reading skills because adults are more likely to read to them regularly.', 'Many of the books in the homes of the high achieving readers from the study were advanced science textbooks that were stored out of reach of the children.']