Which sentence is correctly punctuated?
OA. The graduating seniors collected their gowns, caps, and diplomas before heading to the auditorium.
OB. The graduating seniors collected their gowns caps and diplomas, before heading to the auditorium.
OC. The graduating seniors collected their gowns caps, and diplomas before heading to the auditorium.
O D. The graduating seniors collected their gowns, caps, and diplomas; before heading to the auditorium.