This question refers to the figure shown.Which one of the following concepts explains why heavy nuclei do not follow the N = Z line (or trend) in the figure? 
Definitions:
Total Utility
The total satisfaction received from consuming a certain amount of goods or services.
Marginal Rate of Substitution
The rate at which a consumer is willing to give up one good in exchange for another good, while keeping overall utility constant.
Substitution Effect
A shift in consumer behavior as a result of alterations in the comparative costs of products, resulting in the replacement of one product for another.
Indifference Curves
Graphical representations in economics showing different combinations of two goods that give a consumer equal satisfaction and utility, thus marking consumer preferences.
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