A student placed a disk of filter paper in each of the following solutions: disinfectant 1, disinfectant 2, disinfectant 3, and distilled water. While the four disks were soaking in their respective solutions, she streaked a sterile nutrient agar dish with a culture of E. coli bacteria. Then, she placed each disk carefully onto the nutrient agar dish, placed the lid on the dish, taped it shut, and incubated the dish at 37°C for several days. The diagram shows how the nutrient agar dish looked on Day 1 and Day 4. The shaded portion of the plate on day 4 indicates the bacterial growth. What is the control in the experiment?<br />
a. Disk soaked in disinfectant 1<br />
b. Disk soaked in disinfectant 3<br />
c. Disk soaked in disinfectant 2<br />
d. Disk soaked in distilled water