The correct answer is C. Both S1 and S2 are true.
When a metal wire is stretched, its length increases and its cross-sectional area decreases. This change in dimensions causes the resistance of the wire to increase. The change in resistance can be calculated using the following equation:
$R_f = R_o(1 – 2\mu)$
where $R_f$ is the final resistance, $R_o$ is the original resistance, and $\mu$ is the strain.
Strain is a measure of how much the length of the wire changes relative to its original length. It is calculated using the following equation:
$\mu = \frac{\Delta L}{L_o}$
where $\Delta L$ is the change in length and $L_o$ is the original length.
The change in resistance due to strain is caused by the change in the cross-sectional area of the wire. When the wire is stretched, its cross-sectional area decreases. This decrease in cross-sectional area causes the current density to increase. The current density is the current per unit area of the wire. The increase in current density causes the resistance of the wire to increase.