Choose the correct answer and explain why the other ones are incorrect.

The graph below shows the effect of substrate concentration on the action of enzyme X. This enzyme is functioning at its optimal temperature, 36°C, and at its optimal pH, 5.5.

When the substrate concentration increases from 0.4% to 0.5%, the rate of the reaction

A. decreases

B. increases

C. remains the same

D. increases, then decreases