Choose the word or phrase among A, B, C or D that best fits the blank space in the following passage
There is now increasing concern about the world’s energy (23)………………, particularly about those involving fossil (24)………………. In less than a hundred years we shall probably use up all the present (25) ……………… of oil and gas. The world’s coal reserves should last longer but, once used, these cannot (26)……………… It is important, therefore, that we should develop such (27)……………… sources of energy as solar energy as well as water and wind power, classed as renewable energy.
(23)....................
