The repeat unit of nylon 2-nylon 6 is $-[\mathrm{NH}-\mathrm{CH}{}_2-\mathrm{CO}-\mathrm{NH}-(\mathrm{CH}{}_2){}_5-\mathrm{CO}]{}^{-}$. Its molar mass (C 12, H 1, N 14, O 16) is

1. A152 g/mol
2. B188 g/mol
3. C170 g/mol
4. D182 g/mol