(a) $SH^->OH^->H_2O>OAC^->PHO^-$ (b) $SH^->OH^->PHO^->OAC^->H_2O$ (c) $SH^->OH^->H_2O>PHO^->OAC^-$ (d) $SH^->OH^->ACO^->H_2O>PHO^-$