$A$ student carries out an experiment and plots the $V-I$ graph of three samples of nichrome wire with resistances $R_1, R_2$ and $R_3$ respectively (Figure). Which of the following is true?

When a $12\, V$ battery is connected across an unknown resistor,there is a current of $2.5\, mA$ in the circuit. Find the value of the resistance of the resistor.

In the circuit diagram shown,the two resistance wires $A$ and $B$ are of the same area of cross-section and same material,but $A$ is longer than $B$. Which ammeter,$A_{1}$ or $A_{2}$,will indicate a higher reading for current? Give a reason.

$A$ piece of wire is redrawn,without change in volume,so that its radius is halved. Compare the new resistance with the original resistance.

Define resistance of a conductor. State the factors on which resistance of a conductor depends. Name the device which is often used to change the resistance without changing the voltage source in an electric circuit.
Calculate the resistance of $50 \, m$ length of wire of cross-sectional area $0.01 \, mm^2$ and of resistivity $5 \times 10^{-8} \, \Omega \, m$.

$(a)$ Explain why a conductor offers resistance to the flow of current.
$(b)$ Differentiate between conductor,resistor,and resistance.