Based on Fig. P7.82, how much elastic potential energy is stored in the DNA when it is stretched $50 \mathrm{~nm}$ ? (a) $2.5 \times 10^{-19} \mathrm{~J}$; (b) $1.2 \times 10^{-19} \mathrm{~J}$; (c) $5.0 \times 10^{-12} \mathrm{~J}$; (d) $2.5 \times 10^{-12} \mathrm{~J}$.