Answer: B. low mass nuclei are combined into heavier ones, releasing energy in the process
Step-by-step explanation:
Nuclear fusion is a process which involves the conversion of two small nuclei to form a heavy nuclei along with release of energy.
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Nuclear fission is a process which involves the conversion of a heavier nuclei into two or more small and stable nuclei along with the release of energy.
For Example:
Thus if a heavier nuclei is being formed from two smaller nuclei releasing energy this is a nuclear fusion reaction.