On which factors does the resistivity of a conducting wire depend?

$A$ 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?

$(a)$ Resistivity of iron is $10 \times 10^{-8} \; \Omega m$ and of mercury is $94 \times 10^{-8} \; \Omega m$. Which among iron and mercury is a better conductor? Give reason.
$(b)$ Resistivity of a material $A$ is in the range of $10^{10}-10^{14} \; \Omega m$ while that of $B$ is in the range of $10^{-6}-10^{-8} \; \Omega m$. Which of the two will behave as an insulator and why?

The voltage-current variation of two metallic wires $X$ and $Y$ at constant temperature are shown below. Assuming that the wires have the same length and the same diameter, explain which of the two wires will have larger resistivity.

What happens to the resistance of a conductor when the length of the conductor is reduced to half?

What is the resistance of an ideal ammeter?