When two intermediate chemical equations are combined, the same substance that appears in the same phase can be canceled out, provided that

a) it is a reactant in one intermediate reaction and a catalyst in the other reaction.
b) it is a product in one intermediate reaction and a catalyst in the other reaction.
c) it is a reactant in one intermediate reaction and a product in the other reaction.
d) it is a reactant in both of the intermediate reactions.

The correct answer is Option C.

Explanation:

When two intermediate chemical equations combine, the same substance which appears in the same phase on the reactant side can be cancelled out only when it also appears in the same phase on the product side.

For example: The conversion of diamond to graphite follows two intermediate steps, which are written as:

As, carbon dioxide and oxygen gases are present in the same phase and in opposite sides of the chemical reaction. So, it can be cancelled out.

Net reaction follows:  

Hence, the correct answer is Option C.

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