Students in a large statistics class were randomly divided into two
groups. the first group took the midterm exam with soft music
playing in the background while the second group took the exam
with no music playing. the exam scores of the two groups were
then compared.
this experiment was not blind because:

-
students were allowed to keep their eyes open while
taking the exam.

-
the exam was too long.

-
the students knew whether or not music was playing while
they were taking the exam.

-
some of the students did not study for the exam.

-
students were randomized into the two groups.