Vector (Cross) Product question from JEE Main 2004 (AIEEE)

Answer: (c)

1. Cross product is anti-commutative: $\vec{B} \times \vec{A} = -\vec{A} \times \vec{B}$.
2. Given $\vec{A} \times \vec{B} = -\vec{A} \times \vec{B} \Rightarrow 2\vec{A} \times \vec{B} = 0 \Rightarrow AB\sin\theta = 0$.
3. $\sin\theta = 0 \Rightarrow \theta = 0$ or $\pi$; of the options only $\pi$ fits.
4. So the answer is (c) $\pi$.