Which of the following represents a common challenge or disadvantage associated with many alternative energy sources?
A) They are consistently available 24/7, regardless of natural factors.
B) They generally have extremely low initial installation and setup costs.
C) Their output can be intermittent and highly dependent on natural factors like sun or wind.
D) They produce significant amounts of toxic waste that needs long-term storage.
E) They are only suitable for small-scale, individual household use, not for national grids.