The two electrical sub network $N^{1}$ and $N^{2}$ are connected through three resistors as shown in the figure. The voltage across $5 \; \mathrm{ohm}$ resistor and $1 \; \mathrm{ohm}$ resistor are given to be $10 \mathrm{~V}$ and $5 \mathrm{~V}$, respectively. Then voltage across $15 \; \mathrm{ohm}$ resistor is
- $-105 \mathrm{~V}$
- $+105 \mathrm{~V}$
- $-15 \mathrm{~V}$
- $+15 \mathrm{~V}$