Answer: A. Decomposition.
Explanation: This is a reaction when a compound breaks down to form two and/or more easier substances. When this reaction turns into a single replacement, only one element is placed there for basically filling in for another element that is in a compound that is similar to the one before. That being said about the substances, this reaction is a big part because it takes away substances, then replaces them with newer and similar ones each time it has completed it's work. It's just a long train of work for Decomposition.
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