A scientist conducted an experiment to observe the effect of different light intensities on the growth rate of a specific plant species over 10 days. The results are shown in the line graph below. The x-axis represents Light Intensity (lux) and the y-axis represents Average Plant Height Increase (cm).
[Imagine a line graph here: x-axis 0 to 1000 lux, y-axis 0 to 20 cm. Data points: (0, 2), (200, 8), (400, 15), (600, 18), (800, 19), (1000, 19.5)]
Based on the graph, at what light intensity does the plant's growth rate begin to show diminishing returns?
Correct: D
Diminishing returns occur when an increase in the independent variable (light intensity) leads to smaller and smaller increases in the dependent variable (plant height increase). Looking at the graph, the steepest increase in height occurs between 0 and 400 lux (2cm to 15cm). The rate of increase starts to slow down between 400 and 600 lux (15cm to 18cm, a 3cm increase). However, the most pronounced flattening of the curve, indicating significantly diminishing returns, is observed above 800 lux. From 600 to 800 lux, the increase is 1 cm (18 to 19 cm). From 800 to 1000 lux, the increase is only 0.5 cm (19 to 19.5 cm). This shows that further increases in light intensity yield very little additional growth. Among the choices, 'Above 800 lux' best represents this plateauing effect.