The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

One system using solar energy to clean up pollution in water has banks of moving mirrors that focus sunlight onto a waterfall of waste water. The other method reflects light from a mirrored trough onto waste water flowing through glass tubes. The solar purifiers will reduce concentration of organic pollutants to a few parts per billion.
Two methods for water （） with solar energy.