Final answer:
Enzymes can be used repeatedly because they catalyze reactions without being consumed. They are sensitive to temperature and pH but function efficiently under optimal conditions. Therefore, the correct option is C.
Step-by-step explanation:
Enzymes can be used over and over again because they can catalyze reactions without being consumed. Unlike other catalysts, enzymes are highly specific for particular chemical reactions, generally catalyzing only one or a few types of reactions. They help the reactants interact by binding with them and properly orienting them for the reaction but are not used up in the process. Therefore, after a reaction takes place, enzymes can be regenerated and repeat their catalytic action.
Although enzymes are incredibly efficient and can increase reaction rates enormously, they are sensitive to temperature and pH levels. High temperatures and pH extremes can denature enzymes, causing them to lose their three-dimensional structure and, therefore, their function. However, under optimal conditions, enzymes maintain their structure and can continue to facilitate biochemical reactions without being depleted.