b
\(v =\frac{2 r ^{2}\left(\rho-\rho_{0}\right) g }{9 \eta}\)

\(v \propto \frac{\left(\rho-\rho_{0}\right)}{\eta}\)

\(\frac{ v _{2}}{ v _{1}}=\frac{\rho-\rho_{ g }}{\eta_{ g }} \times \frac{\eta_{ w }}{\rho-\rho_{ w }}\)

\(\frac{ v _{2}}{10}=\frac{7.8-1.2}{13.2} \times \frac{8.5 \times 10^{-4}}{7.8-1}=0.625 \times 10^{-4}\)

\(v _{2}=10 \times 0.625 \times 10^{-4}\)

\(v _{2}=6.25 \times 10^{-4} \;cm / s\)