Microbial fuel cells are considered a source of sustainable energy. Why?
1. They use living organisms as catalysts to generate electricity from certain substrates.
2. They use a variety of inorganic materials as substrates.
3. They can be installed in waste water treatment plants to cleanse water and produce electricity.
Which of the statements given above is/are correct? [UPSC 2011 (CS-P)]

1 only
2 and 3 only
1 and 3 only
1, 2 and 3