In these questions, the letters are linked in some way. Identify the relation and complete the last box. \[\text{7}\,\,\,\text{:}\,\,\,\,\text{21}\] \[\text{8}\,\,\,\text{:}\,\,\,\,\text{24}\] \[\text{3}\,\,\,\text{:}\,\,\,\,\text{?}\]

• A.  6                            
• B.         9            
• C.  12                                         
• D.  7