As shown, two resistors joined in series form a potential divider. Their resistances are $R_1$ and $R_2$, and the potential differences across them are $V_1$ and $V_2$ respectively. Which expression is equal to $\frac{V_1}{V_2}$?
- A$\frac{R_1}{R_2}$
- B$\frac{R_2}{R_1}$
- C$\left(\frac{R_1}{R_2}\right)^2$
- D$\left(\frac{R_2}{R_1}\right)^2$