Sulfur tetrafluoride, SF₄, has an expanded octet around sulfur. In a structure where sulfur is bonded to four fluorine atoms (two axial, two equatorial) with single bonds and possesses one lone pair, which set of formal charges is correct for the molecule?
a) S:+1; each F:−21
b) S:0; each F:0
c) S:+2; each F:−1
d) S:+1; one axial F:−1; other atoms 0