Cr orthogonality

1. A.For $c_1,c_2\in \mathbb{C}$, the level curves $u=c_1$ and $v=c_2$ are orthogonal wherever they intersect.
2. B.$\nabla u\cdot \nabla v=0$ at every point.
3. C.If f is an entire function, then $\nabla u\cdot \nabla v=0$ at every point.✓
4. D.If $\nabla u\cdot \nabla v=0$ at every point, then f is an entire function.

The trap it tests

Execution slip

The idea was right. The computation was not.