Silicon has a diamond-cubic cell of edge $5.43\times 10^{-8}\mathrm{cm}$ containing 8 Si atoms. It is doped with phosphorus at 1 P atom per $10^6$ Si atoms. Each P donates one free electron. The free-electron concentration is nearest to

1. A$5.0\times 10^{22}\mathrm{c}{\mathrm{m}}^{-3}$
2. B$2.5\times 10^{16}\mathrm{c}{\mathrm{m}}^{-3}$
3. C$0.6\times 10^{16}\mathrm{c}{\mathrm{m}}^{-3}$
4. D$5.0\times 10^{16}\mathrm{c}{\mathrm{m}}^{-3}$