From which of the following studies can cause-and effect conclusions potentially be drawn? If cause and effect can be determined, explain what may cause what.
a. From a random sample of city residents, it was found that those with higher incomes utilize the recycling services significantly more than those with lower incomes.
b. Students were randomly assigned to two groups. One group listened to music while taking a math test and one group did not. The one that did not listen to music scored significantly higher on the math test than the other.
c. A teacher gave one of his classes a math test while music was playing and another without music playing. The class that did not listen to music scored significantly higher on the math test than the other.