Which statement concerning the nuclide notation ${}^R_Z X$ is correct?
- A$R$ is the nucleon number which is different for different isotopes of $X$.
- B$R$ is the proton number which is the same for different isotopes of $X$.
- C$Z$ is the nucleon number which is the same for different isotopes of $X$.
- D$Z$ is the proton number which is different for different isotopes of $X$.