The photoelectron spectrum for the element boron is represented above. Which of the following best explains how the spectrum is consistent with the electron shell model of the atom?

$($A$)$ The spectrum shows an odd number electrons.

$($B$)$ The spectrum shows a single electron in the $2p$ subshell.

$($C$)$ The spectrum shows equal numbers of electrons in the first and second electron shells.

$($D$)$ The spectrum shows three electrons with the same binding energy in the second electron shell.