Which one is a decomposition reaction?

• A.  \[2HgO\xrightarrow{heat}2Hg+{{O}_{2}}\]
• B.  \[CaC{{O}_{3}}\xrightarrow{heat}CaO+C{{O}_{2}}\]
• C.  \[2{{H}_{2}}O\xrightarrow{Electrolysis}2{{H}_{2}}+{{O}_{2}}\]
• D.  All the above reactions are decomposition reaction