【单选题】The need for solar electricity is clear. It is safe, ecologically sound, efficient, continuously available, and is has no moving parts. The basic problem with the use of solar photovoltaic devices is ...
A.
discussing the importance of solar energy
B.
explaining the functioning of solar cells
C.
presenting a history of research on energy sources
D.
describing a possible solution to the problem of the cost of photovoltaic cells