An electric dipole with dipole moment $\vec{p}_i = (3\hat{i} + 4\hat{j}) \times 10^{-30} \, \text{C-m}$ is placed in an electric field $\vec{E} = 4000 \hat{i} \, \text{N/C}$. An external agent turns the dipole slowly until its electric dipole moment becomes $\vec{p}_f = (-4\hat{i} + 3\hat{j}) \times 10^{-30} \, \text{C-m}$. The work done by the external agent is equal to:

• A
  $4 \times 10^{-28} \, \text{J}$
• B
  $-4 \times 10^{-28} \, \text{J}$
• C
  $2.8 \times 10^{-26} \, \text{J}$
• D
  $-2.8 \times 10^{-26} \, \text{J}$