If two vectors $\vec{A}$ and $\vec{B}$ have equal magnitude $R$ and are inclined at an angle $\theta$, then which of the following is correct?

• A
  $|\vec{A}-\vec{B}|=2 R \sin \left(\frac{\theta}{4}\right)$
• B
  $|\vec{A}+\vec{B}|=2 R \sin \left(\frac{\theta}{2}\right)$
• C
  $|\vec{A}+\vec{B}|=2 R \cos \left(\frac{\theta}{2}\right)$
• D
  $|\vec{A}-\vec{B}|=2 R \cos \left(\frac{\theta}{2}\right)$