Two horizontal parallel-plate conductors are spaced $20\ \text{mm}$ apart in air. The upper plate is earthed, and the lower plate has a potential of $+100\ \text{V}$. What is the electric field strength at point $P$ halfway between the plates?
- A$5000\ \text{V m}^{-1}$ downwards
- B$5000\ \text{V m}^{-1}$ upwards
- C$10000\ \text{V m}^{-1}$ downwards
- D$10000\ \text{V m}^{-1}$ upwards