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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Fission is the opposite of fusion and releases energy only when heavy nuclei are split. As noted in Fusion, energy is released if the products of a nuclear reaction have a greater binding energy per nucleon ( BE / A ) than the parent nuclei. Figure 32.25. Answer: fission
Output:
What process is the opposite of fusion and releases energy only when heavy nuclei are split?