A substance that influences the reaction but does not participate in the reaction is

a catalyst

Explanation:

Catalyst is a substance that influences the rate of reaction without being consumed in the reaction. They increase the rate of reaction by decreasing the activation energy of the reaction. In biological system, enzymes are referred to as biological catalyst or biocatalyst. They are generally protein in nature and are very specific for their substrate. They also increases the rate of biological reaction by forming enzyme-substrate complex.

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plato users:

d.  

there was a variety of food available on island x, but there were only large seeds and nuts on island y.