The mass number of an element is equal to:
a
the number of protons
b
the number of protons and electrons
c
the number of electrons
d
the number of protons and neutrons
d
the number of protons and neutrons
a
the number of protons
b
the number of protons and electrons
c
the number of electrons
d
the number of protons and neutrons
the number of protons and neutrons