Which of the following pairs represents linkage isomers?

1. A
   $[\text{Cu}({\text{NH}}_3{)}_4][{\text{PtCl}}_4]$ and $[\text{Pt}({\text{NH}}_3{)}_4][{\text{CuCl}}_4]$
2. B
   $[\text{Pd}({\text{PPh}}_3{)}_2(\text{NCS}{)}_2]$ and $[\text{Pd}({\text{PPh}}_3{)}_2(\text{SCN}{)}_2]$
3. C
   $[\text{Co}({\text{NH}}_3{)}_5{\text{NO}}_3]{\text{SO}}_4$ and $[\text{Co}({\text{NH}}_3{)}_5{\text{SO}}_4]{\text{NO}}_3$
4. D
   $[{\text{PtCl}}_2({\text{NH}}_3{)}_2]{\text{Br}}_2$ and $[{\text{PtBr}}_2({\text{NH}}_3{)}_2]{\text{Cl}}_2$