Elena is conducting an experiment to determine if the high school students are more relaxed when the lights are off or when music is played during an exam. She selects 10 of her friends to take an exam with the lights off and another 10 of her friends to take an exam with music playing. Required:
What is problematic about the way that Elena selected her groups?

Problematic is that - only one factor should be analysed at a time. However she has considered both.

Step-by-step explanation:

Groups for any experimental study should be selected on the basis of one variable, other factor(s) remaining constant.

In this case, here are two variables - lights on or off, music playing or not playing - effecting relax level during exams.

She selects 10 students for exam with lights off , 10 for exam without music. This is problematic, as at once only lights or only music factor should be considered.

If one group is with light on , other should be with light off. If one group is with music, other should be without music. This mix up of one light off aspect & one music without music is inapt.