Why do we say that "an enzyme is reusable"?
a) the enzyme does not actively take part in the reaction. it just indicates that the reaction is complete.
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b) enzymes react with the substrate molecules to form products and become denatured in the process.
c) enzymes ensure that the reaction continues without inhibition even if conditions such as temperature and ph are changed.
d) the products of the reaction are released from the active sites of the enzyme, allowing other substrate molecules to bind with the sites.

The correct option is D) The products of the reaction are released from the active sites of the enzyme, allowing other substrate molecules to bind with the sites. 

Enzymes are protein molecules which have a specific shape complimentary to the substance they act on, known as the substrate. The region where the reaction occurs is the enzyme's active site. However, when the reaction is complete, the products leave the active site and the enzyme is once again available for reaction. This means that the enzyme is unchanged through the reaction. This is why enzymes are known as biological catalysts.