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 opener. While carefully monitoring the changes in the heart rate 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:_________.
1. adoptation level phenomenon
2. relative deprivation principle
3. guilty knowledge test
4. catharsis hypothesis