单项选择题

案例分析题As concerns over man’s impact on the global environment increase, many interesting innovations are being considered. For example, "green roofs" - plant-filled roof top gardens - are one substantial way to confront the country’s ecological problems, and there seems to be very little in the way of a downside. For one thing, a green roof acts as a sponge for rainwater, absorbing the majority of water from a typical rainstorm. Only three to five inches of soil for a green roof is sufficient for this to occur, which keeps the weight of the roof to a minimum. Because the plantings on the roof absorb the rainwater, drainage and sewage systems have a decreased volume of water pumping through them, keeping these systems from becoming over-stressed. And while, the soil and plants are absorbing rainwater, they are also taking in pollutants for their own nourishment by storing carbon and then emitting necessary oxygen back into the atmosphere.
The author’s main point in the passage is that （）