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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?

Created for Albert.io by Jennie Edgar. Copyright 2016.

A

An appropriate control would be to put the nematodes on a Petri dish without any drops of liquid to determine if the worms are attracted to one side of the choice chamber or not.

B

An appropriate control would be to put the nematodes on a Petri dish with salt water to see the effect of salt water on the behavior of C. elegans.

C

An appropriate control would be to put the nematodes on a Petri dish with just plain water on both sides to determine if liquid was an attractant to the worms.

D

An appropriate control group is to add the same amount of liquid to both sides of the Petri dish to maintain a constant level of liquid in the experiment.

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