Circle all groups that represent isotopes a.
_(7)^(14)x,_(6)^(14)x
b.
,_(7)^(14)x,_(6)^(13)x
c.
,_(7)^(14)x,_(7)^(15)x
d.
,_(6)^(12)x,_(6)^(13)x,_(6)^(14)x
e.
^(16)x,_(7)^(16)x
f.
,_(8)^(16)x,_(8)^(15)x