A graph shows substrate concentration on the x axis and rate of enzyme reaction on the y axis. Line labeled uninhibited rises steeply from the 0 point and then levels off at a high rate of enzyme reaction before moving horizontally to end of graph. Line B starts at 0 and rises gradually to meet uninhibited line at high rate of reaction at end of graph. Line A starts at 0, rises gradually to about halfway up y axis then moves horizontally to end of graph. The image shows the rate of an enzyme reaction under conditions of no inhibition, competitive inhibition, and noncompetitive inhibition as reactions labeled uninhibited, A, and B. Which of the following best explains what has occurred in the enzyme reactions?