Although newspapers reported that a murder victim had been stabbed with a knife, two police investigators knew that the actual murder weapon was a letter openor. While carefully monitoring the changes in heart and perspiration level of a prime suspect, the investigators asked him if he typically used a letter opener on his mail. The investigators were making use of the:

A. Relative deprivation principle.
B. Guilty knowledge test
C. Adaptation-level phenomenon
D. Two-factor theory