Concept:Nuclear fusion needs extremely high temperatures to force positively charged nuclei together.
Explanation:In fusion, light nuclei like hydrogen combine to form a heavier nucleus and release great energy.
This process can occur only when the nuclei overcome their strong electric repulsion.
That requires temperatures of millions of degrees, like those inside the Sun.
Such extreme conditions are very difficult to create and contain on Earth.
Therefore, fusion is not used as a practical energy source because very high temperature is needed for the reaction.
Answer:A. very high temperature is needed for the reaction