Which of the following involves combination of two elements?

• A. \[CaO+C{{O}_{2}}\to CaC{{O}_{3}}\]
• B. \[4Na+{{O}_{2}}\to 2N{{a}_{2}}O\]
• C. \[2S{{O}_{2}}+{{O}_{2}}\to 2S{{O}_{3}}\]
• D. \[N{{H}_{3}}+HCl\to N{{H}_{4}}Cl\]