Which of the following best represents an absolute threshold?

a. A guitar player knows that his D string has just gone out of tune.

b. A photographer can tell that the natural light available for a photograph has just faded slightly.

c. Your friend amazes you by correctly identifying unlabeled glasses of Coke and Pepsi.

d. A cook can just barely taste the salt she has added to her soup.

e. Your mom throws out the milk because she says the taste is “off.”