An electric dipole is placed in a uniform electric field $\vec{E}$ of magnitude $40\ N/C$. The graph shows the magnitude of the torque $\tau$ on the dipole versus the angle $\theta$ between the field $\vec{E}$ and the dipole moment $\vec{p}$. The magnitude of the dipole moment $\vec{p}$ is equal to:

• A
  $1.25 \times 10^{-28}\ C\cdot m$
• B
  $2.0 \times 10^{-25}\ C\cdot m$
• C
  $2.5 \times 10^{-28}\ C\cdot m$
• D
  $5.0 \times 10^{-28}\ C\cdot m$