All except one of the following demonstrate a difference threshold. The exception represents an absolute threshold. Which of the following represents an absolute threshold?
1) a guitar player knows that his D string has just gone out of tune
2) a photographer can tell that the natural light available for a photograph has just faded slightly
3) your friends amazes you by correctly identifying unlabeled glasses of Coke and Pepsi
4) a cook can just barely taste the salt she has added to her soup
5) your mom throws out the milk because she says the taste is "off"