In a police line-up, the witness is asked to identify one criminal from a group of similar-looking people. If the authorities who conduct the line-up do not know which person is the suspected criminal, and the witness does not know either, then which experimental procedure is a police line-up most like?
a. A single-blind experiment.
b. A double-blind experiment.
c. A non-standard standard scientific experiment.
d. A field experiment.