Before solar energy can become a competitive form of energy available for consumers, which of the following events must occur? a. solar energy is used to power a photovoltaic cell, which splits water into hydrogen and oxygen, and the hydrogen can be used to provide energy. b. solar energy passes through a photovoltaic cell, which then stores the energy as heat until it is needed. c. energy must be converted into another form, stored until needed, and then converted into heat. d. energy must be collected, converted into another form, and stored until needed.