The $I-V$ characteristics of the zener diodes $D1\:\text{and}\:D2$ are shown in Figure I. These diodes are used in the circuit given in Figure II. If the supply voltage is varied from $0$ to $100$ V,then breakdown occurs in
- $D1$ only
- $D2$ only
- both $D1$ and $D2$
- none of $D1$ and $D2$