After playing tennis, Briana hurries home because her mother always bakes Briana's favorite scones on Saturday afternoons. As Briana opens the front door, she notices the powerful odor of a gas leak, but her mother says, "I don't smell anything." It is likely that Briana's mother has:
a. experienced a decline in her sense of smell as she reached middle age.
b. has a different absolute threshold for olfaction than does her daughter.
c. adapted to the smell of the trashcan because it is an unchanging stimulus.
d. is a naysayer, while Briana is a yea-sayer.

Absolute threshold for smell has to do with the lowest concentration of a gas or odor that can be perceived by an individual. The lower the threshold, the more an individual can perceive an odor. It can be concluded that Briana has a lower threshold for olfaction than her mum thus she was able to perceive the gas leak. Note that there is an increased possibility for the concentration of the gas to be steady as it leaks at a steady rate and also diffuses at a steady rate.

Option C is wrong because, the smell of gas is what is considered and not trashcan.

Option D is totally out of context.

Option A is wrong because scientifically, the sense of smell begins to decrease at age 60 and above.