The graph illustrates how substrate concentration affects the rate of an enzyme-controlled reaction. The results were obtained with a constant enzyme concentration at a temperature of 30^{\circ}C. Which statement explains the graph’s shape from X to Y?
- ADenaturation of the enzyme molecules is increasing.
- BThe enzyme active sites are not all occupied.
- CThe concentration of substrate molecules is decreasing.
- DThe frequency of effective collisions between enzyme active sites and substrate molecules is decreasing.