A chemical reaction occurred because calcium oxide has different properties than calcium and diatomic oxygen. The correct option is A.
A chemical reaction is said to occur when the chemical properties of substances change during the course of the reaction.
From the illustration, the chemical properties of calcium oxide are unique from that of calcium and oxygen. Thus, calcium oxide has become a new substance entirely.
Once a reaction leads to the formation of new substances, such a reaction is a chemical reaction.
More on chemical reactions can be found here: https://brainly.com/question/22817140
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