In the voltage regulator shown in the given figure, the load current can vary from $100 \mathrm{~mA}$ to $500 \mathrm{~mA}$. Assuming that the Zener diode is ideal (i.e., the Zener knee current is negligibly small and Zener resistance is zero in the breakdown region), the value of $R$ is
- $7 \; \Omega$
- $70 \; \Omega$
- $\frac{70}{3} \; \Omega$
- $14 \; \Omega$