Two broad horizontal metal plates are spaced $4\,\text{mm}$ apart. The lower plate is held at a potential of $-80\,\text{V}$. What potential must be applied to the upper plate so that the electric field between the plates has a strength of $60\,000\,\text{V m}^{-1}$ upwards?
- A$-320\,\text{V}$
- B$-160\,\text{V}$
- C$+160\,\text{V}$
- D$+320\,\text{V}$