Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas  In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide
Plants in the urban greenhouse grew more slowly than those in the rural greenhouse
The urban class conducting the experiment was larger than the rural class conducting the experiment.