Which of the following hypotheses about why the noinhibitor graph plateaus after 10% H₂O₂ is supported?
(A) Water begins to act as an inhibitor.
(B) All the HRP is fully engaged.
(C) There is no more H₂O₂ available.
(D) The temperature is no longer optimal.

Hydrogen peroxide (H₂O₂) is broken down into oxygen and water by the enzyme horseradish peroxidase (HRP). This reaction can be measured with an oxygen-sensitive color indicator, and the color change can be measured on a spectrophotometer. The rate of oxygen formation is measured with a constant HRP concentration and 5 different concentrations of H₂O₂ at room temperature. A follow-up experiment adds a known inhibitor of HRP (sodium azide). The results of both studies are shown in Figure 1.

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Velocity 1 0.8 0.6 0.4 0.2 0 0 2.50% 5% [H₂O₂] 10% 100%