Let the functions $u$ and $v$ be differentiable functions of $x$, $y$ and $z$. Show that a necessary and sufficient condition that $u$ and $v$ are functionally related by the equation $F(u,v)=0$ is
- $\bigtriangledown u \times \bigtriangledown v =0$
- $\bigtriangledown u . \bigtriangledown v =0$
- Both A and B
- None of the above