During sterilization of a fermentation medium in a given bioreactor $\nabla_{heaing}=12.56, \nabla_{cooling}= 7.48$ and the total value of $\nabla$ required for whole sterilization process is $52$, where $\nabla$ is the design criteria.

What is the value of $\nabla_{holding}$?

1. $31.96$
2. $42.32$
3. $52.43$
4. $61.18$

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