The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

Many scholars argue that the world is heading towards a global energy crisis mostly from running out of cheap oil and recommand decreasing dependency on fossil fuel. This has caused many to invest in alternate fuel research such as fuel cell technology, hydrogen fuel, methanol, solar energy, tidal energy and wind energy. But so far, only hydroelectricity and nuclear power have seen significant usage.
Future and （） sources of energy.