A nucleus of the isotope potassium-40 decays to a nucleus of the isotope argon-40. The reaction equation for this decay may be written as

\[_{{\text{19}}}^{{\text{40}}}{\text{K}} \to _{\;Z}^{{\text{40}}}{\text{Ar}} + {\text{X}} + \nu \]

Which of the following correctly identifies the proton number of argon-40 and the particle X?

B