Buta-1,3-diene reacts with one equivalent of HBr at $40{}^{\circ }\mathrm{C}$ and the products are allowed to equilibrate (thermodynamic control). The major product is

1. A$\mathrm{CH}{}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{}_2\mathrm{Br}$ (1-bromobut-2-ene)
2. B$\mathrm{CH}{}_3-\mathrm{CH}=\mathrm{CBr}-\mathrm{CH}{}_3$ (2-bromobut-2-ene)
3. C$\mathrm{CH}{}_3-\mathrm{CHBr}-\mathrm{CH}=\mathrm{CH}{}_2$ (3-bromobut-1-ene)
4. D$\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Br}$ (1-bromobutane)