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?

1. (a)1 only
2. (b)2 and 3 only
3. (c)1 and 3 only
4. (d)1, 2 and 3