The graph illustrates how one physical quantity $y$ changes with another physical quantity $x$. Which row lists suitable quantities for $y$ and $x$?
- A$y$: the number of atoms of a radioactive isotope present; $x$: the time taken
- B$y$: the potential difference across a metallic conductor; $x$: the current in the metallic conductor
- C$y$: the resistance of a length of wire; $x$: the diameter of the wire
- D$y$: the volume of a $1.0\,\text{kg}$ object; $x$: the density of the material from which the object is made