What is a superconductor?

The given graph shows the variation of resistance of mercury in the temperature range $\mathrm{O}< \mathrm{T}< 4\mathrm{K}.$ Name the phenomenon shown by the graph of figure.

$\mathrm{I}-\mathrm{V}$ graph for parallel and series combination of two metallic resistors are shown in figure. Which graph represents parallel combination? Justify your answer.

A set of $n$ identical resistors, each of resistance $R \mathrm{ohm}$, when connected in series have an effective resistance $X \mathrm{ohm}$ and when the resistors are connected in parallel, their effective resistance is $Y \mathrm{ohm}$. Find the relation between $R, X$ and $Y$.