Rotation about the $\mathrm{C}{}_2-\mathrm{C}{}_3$ bond of n-butane produces several conformations. The conformation of highest energy is the one in which

1. Athe two $\mathrm{CH}{}_3$ groups are anti (dihedral angle $180^{\circ }$)
2. Bthe two $\mathrm{CH}{}_3$ groups are gauche (dihedral angle $60^{\circ }$)
3. Ceach $\mathrm{CH}{}_3$ is eclipsed with a hydrogen (dihedral angle $120^{\circ }$)
4. Dthe two $\mathrm{CH}{}_3$ groups are eclipsed (dihedral angle $0^{\circ }$)