A:1s22s22p63s23p3→1s22s22p63s23p2
B:1s22s22p63s23p2→1s22s22p63s23p1
C:1s22s22p63s23p4→1s22s22p63s23p3
D:1s22s22p63s23p64s24p2→1s22s22p63s23p64s24p1