During the experimental exposure of algae to Paramecium predators, traits other than multicellularity presumably could have also evolved as a response to selection. What measure taken by the authors allows them to more confidently conclude that multicellularity, and not other traits that may have also evolved in their experimental populations (e.g., toxicity), is the main adaptation against predation? 

Question 6 options: 

They used rotifers instead of Paramecium in their test of predation because the algae would likely not have evolved adaptations against predation by rotifers. 

They tested if multicellular algae were more toxic than unicellular algae. 

They did not address this potential problem. 

They compared every possible trait between the control and the experimental populations and found that multicellularity was the only difference between the two groups.