$9\left(R^{3}-r^{3}\right)=R^{3}$
$r^{3}=\frac{8 R^{3}}{9}$

Density of whole blood, $\rho=1.06 \times 10^{3} \;kg m ^{-3}$
$(A)$ the net elongation of the spring is $\frac{4 \pi R^3 \rho g}{3 k}$
$(B)$ the net elongation of the spring is $\frac{8 \pi R^3 \rho g}{3 k}$
$(C)$ the light sphere is partially submerged.
$(D)$ the light sphere is completely submerged.