Volunteers for a human performance study were randomly divided into two groups. The first group had their flexibility measured in the morning after a short meditation session while the second group had their flexibility measured in the afternoon with no previous meditation session. The flexibility scores of the two groups were compared. This experiment was not double-blind because: a. some of the volunteers stretched before they went to the study location.
b. volunteers were allowed to keep their eyes open while having their flexibility measured.
c. volunteers were randomized into the two groups.
d. the volunteers knew whether they had meditated before having their flexibility measured.