An oil drop carrying negative charge and having mass $m$ is located between two horizontal metal plates that are parallel and separated by a distance $d$. With a potential difference (p.d.) of $V_1$ across the plates, the drop moves upwards with constant speed. If the p.d. is reduced to $V_2$, the drop moves downwards with the same constant speed. Whenever the drop is in motion, air resistance acts on it. The upthrust on the drop is negligible. The acceleration of free fall is $g$. What is the charge on the oil drop?
- A$\frac{mdg}{V_1 - V_2}$
- B$\frac{mdg}{V_1 + V_2}$
- C$\frac{2mdg}{V_1 - V_2}$
- D$\frac{2mdg}{V_1 + V_2}$