A、They need little water. B、They normally grow faster. C、They are less costly to grow. D、They could absorb more water. D文中说：与景天属

Green roofs are designed to save energy and capture rainwater. Rooftops covered with plants help keep buildings cool. They can extend the life of a roof. And they can reduce water runoff into streets and storm water systems.
    Several years ago, the mayor of Chicago, Illinois, visited Germany, a leader in green roof design. Mayor Richard Daley later decided to have vegetation planted on Chicago City Hall. Today, the tops of many other buildings also look like parks.
    In Canada, the Coast Plaza Hotel in Vancouver has a forest on its roof. Experts say green roofs usually do not get much above twenty-seven degrees Celsius. So temperatures might not be a problem for growing. But rooftop gardeners have to be careful not to put too much weight on the roof. This is true whether plants are grown in soil or water.
    New York City has an unusual demonstration farm. The farm is on a boat called the Science Barge. Rainwater and purified river water are used to grow tomatoes, cucumbers and peppers. The farm shows the possibilities for designing rooftop systems.
    Earlier this year, researchers in Texas reported that green roofs can reduce a building’s air conditioning costs by about one-fifth compared to black-top roofs. But they also reported that not all green roofs perform equally well. They found that designs differ in their ability to keep buildings cool and to capture rainwater. The presence of native plants likely helped all the green roofs capture water better compared to sedums. Sedums are plants that need little water and often are used on green roofs. The native plants could take in more water and release more of it to the atmosphere.
Q13: What might be a problem for growing rooftop plants?
Q14: Which of the following is true about green roofs?
Q15: What is the advantage of native plants over sedums?