C. elegans, a nematode or "round worm," is a model organism in biology. A group of AP biology students was investigating associative learning in C. elegans. C. elegans feeds on bacteria, a particular strain of E. coli OPI150.
For one week, when students added E. coli OPI150 to the nematode culture, they also added salt ($NaCl$). Then, students placed the nematodes on a Petri dish with two choices, one mL of $NaCl$ solution and one mL of water, as seen in the image below.
Which of the following identifies an appropriate control group in this experiment?