What was the objective of the Franck and Hertz experiment?

To measure the ionization potentials of atoms.
To study the behavior of electrons in a vacuum.
To observe the emission spectra of mercury vapor.
To measure the energy levels of hydrogen atoms.
a) To measure the ionization potentials of atoms.
b) To study the behavior of electrons in a vacuum.
c) To observe the emission spectra of mercury vapor.
d) To measure the energy levels of hydrogen atoms.