Which one among the following is a double salt? [NDA]

• A. \[{{K}_{4}}[Fe{{(CN)}_{6}}]\]
• B. \[{{K}_{2}}S{{O}_{4}}.A{{l}_{2}}{{(S{{O}_{4}})}_{3}}.24{{H}_{2}}O\]
• C. \[CuS{{O}_{4}}.5{{H}_{2}}O\]
• D. \[NaCl\]