Which of the Following Steps Do Christopher Flavin and Seth

Which of the following steps do Christopher Flavin and Seth Dunn recommend as supportive of more sustainable generation of energy in the future?

A) wider use of a semiconductor device known as a solar voltaic cell
B) burning more natural gas and less oil
C) development of the fuel cell, an electrochemical process combining hydrogen and oxygen to produce water and electricity
D) all of the above
E) none of the above