The right response is combustion reaction and synthesis reaction.
A hydrocarbon reacting chemically with oxygen to produce carbon dioxide and water is known as a combustion reaction.
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The term "synthesis reaction" refers to the chemical process wherein elements combine to generate a single product.
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Because hydrogen and chlorine combine to form the molecule of hydrochloric acid in their elemental states, this process is regarded as a synthesis reaction.
For A :
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Because hydrogen gas and chlorine gas are mixed in their elemental states to create the hydrochloric acid molecule, this process is regarded as a synthesis reaction.
For B:
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Because methane and oxygen combine to create carbon dioxide and water molecules, this reaction is regarded as a combustion reaction.
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