Alma wants to prove to her parents that listening to music while she studies for her tests will her get better grades. she decides to ask 15 of her friends if they listen to music while they study for tests and what their grades are. she finds that her friends who have high grades also listen to music while they study for tests. she tells her parents that she has proven that listening to music while studying for tests will cause her grades to improve. what mistake has alma made?

a) she did not separate her friends into different groups, and she didn't apply a treatment.
b) she did not separate her friends into different groups, and she used a survey to collect data.
c) she did not use a random sample, and she tried to show cause and effect with an experiment.
d) she did not use a random sample, and she tried to show cause and effect with an observational study.