Given the task definition and input, reply with output. Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: A new study found that inner-city kids living in neighborhoods with more green space gained about 13% less weight over a two-year period than kids living among more concrete and fewer trees. Such findings tell a powerful story. The obesity epidemic began in the 1980s, and many people think it results from increased portion sizes and inactivity , but that can't be everything. Big Macs and TVs have been with us for a long time. "Most experts agree that the changes were related to something in the environment," says Thomas Glass of the Hopkins School of Public Health. That something could be a shrinking of the green. The new research, published in the American Journal of Preventive Medicine, isn't the first to associate greenery with better health, but it does get us closer to identifying what works and why. At its most straightforward, a green neighborhood simply means more places for kids to play -- which is important since time spent outdoors is one of the strongest links to children's activity levels. Glass warns that most studies don't necessarily prove a link between greenness and health, but they're helping _ action. In September the U.S. government approved the No Child Left Inside Act to encourage public initiatives aimed at exposing kids to the outdoors. Finding green space is, of course, not always easy, and you may have to work a bit to get your family a little grass and trees. If you live in a suburb or a city with good parks, take advantage of what's there. Your children in particular will love it -- and their bodies will thank you.
Question: According to the passage, what might cause children's inactivity?
a shrinking of the green