Digital ACT Science Practice Question #819

Researchers are investigating the effect of blue light intensity on the growth rate of a specific algae species, *Chlorella vulgaris*. They hypothesize that higher blue light intensity will increase the photosynthetic rate, leading to faster growth. They set up four experimental groups: * **Group 1:** Algae grown in standard white light (control). * **Group 2:** Algae grown in low intensity blue light. * **Group 3:** Algae grown in medium intensity blue light. * **Group 4:** Algae grown in high intensity blue light. All groups are supplied with the same nutrient solution, temperature, and CO2 concentration. Algae growth rate is measured by daily optical density readings. To further refine their understanding of blue light's specific impact, which of the following modifications to the experimental design would BEST isolate the effect of blue light intensity from other spectral components?