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Explanation:
Control rods can then be inserted into the reactor core to reduce the reaction rate or withdrawn to increase it.
To solve the exercise it is necessary to keep in mind the concepts about the ideal gas equation and the volume in the cube.
However, for this case the Boyle equation will not be used, but the one that corresponds to the Boltzmann equation for ideal gas, in this way it is understood that

Where,
N = Number of molecules
k = Boltzmann constant
V = Volume
T = Temperature
P = Pressure
Our values are given as,




Rearrange the equation to find V we have,



We know that length of a cube is given by

Therefore the Length would be given as,



Therefore each length of the cube is 3.44nm
Answer: Sorry the pic is too blurry try making the resolution higher
I think it's the last one.
Hope I helped