Sulphur trioxide can be obtained by which of the following reaction
S + H2SO4 $\xrightarrow{\;\;\triangle\;\;}$
H2SO4 + PCl3 $\xrightarrow{\;\;\triangle\;\;}$
CaSO4 + C $\xrightarrow{\;\;\triangle\;\;}$
Fe2(SO4)3 $\xrightarrow{\;\;\triangle\;\;}$