Copper wire comes in set lengths, but with different diameters $d$. The resistance $R$ changes for each diameter. Which relationship between $R$ and $d$ is correct?
- A$R$ is directly proportional to $d$.
- B$R$ is directly proportional to $d^2$.
- C$R$ is inversely proportional to $d$.
- D$R$ is inversely proportional to $d^2$.