In these questions, the letters are linked in some way. Identify the relation and complete the last box. \[\text{A}\,\,\,\text{:}\,\,\,\,\text{D}\] \[\text{B}\,\,\,\text{:}\,\,\,\,\text{E}\] \[\text{C}\,\,\,\text{:}\,\,\,\,\text{F}\] \[\text{X}\,\,\,\text{:}\,\,\,\,?\]

• A.  A      
• B.  B      
• C.  Z            
• D.         C