问题：
[单选题]Which is the topic sentence in this paragraph?
AThe study authors suggest that green spaces may have a positive effect both directly and indirectly.
B“Green spaces provide children with opportunities to develop mental skills such as discovery and creativity,” says co-author Payam Dadvand, a physician and researcher at the Center for Research in Environmental Epidemiology in Barcelona. More indirectly,
Cgreen spaces may help by reducing exposure to air pollution and noise, increasing physical activity, and enriching microbial input from the environment, all of which have been associated with improved mental development, he says. When the researchers measured and factored in traffic-related air pollution, which is higher in places with fewer plants and trees, they found that it accounted for 20 to 65 percent of the observed association between greenness and cognitive development.
DAir pollution has been shown to have neurotoxic effects, Dadvand says.
