Solid sphere recast into a disc with the same moment of inertia about its rim: radius of the disc

1. A.
   $\frac{2R}{\sqrt{15}}$
2. B.
   $\sqrt{\frac{2}{15}}\,R$
3. C.
   $\frac{4R}{\sqrt{15}}$
4. D.
   $\frac{R}{4}$

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1. A.
   $\frac{1}{\sqrt{2}}$
2. B.
   $\frac{2}{\sqrt{5}}$
3. C.
   $\frac{2}{\sqrt{10}}$
4. D.
   $\frac{2}{\sqrt{15}}$

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