Fission and fusion reactions both release energy. however, only fusion reactions a) require elements with large atomic numbers b) create radioactive products c) use radioactive reactants d) combine light nuclei

D) combine light nuclei

Explanation:
Fission and fusion both deal with the alteration of atoms in order to produce energy. However, they are actually opposite processes, this is because:
1- Fission occurs when an atomic nucleus is split/divided into two forming two atoms
2- Fusion occurs when two light atomic nuclei combine together to form a single atom.

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