\(4.5=\frac{16 \times 10^{-12}}{2 C}\)
\(C=\frac{16 \times 10^{-12}}{9}=4 \pi \varepsilon_{0} R\)
(capacity of spherical conductor)
\(\mathrm{R}=\frac{16 \times 10^{-12}}{9} \times \frac{1}{4 \pi \varepsilon_{0}}\)
\(\because \frac{1}{4 \pi \varepsilon_{0}}=9 \times 10^{9}\)
\(=9 \times 10^{9} \times \frac{16}{9} \times 10^{-12}=16\, \mathrm{mm}\)