Behavioral psychologists apprehend that conditioned fear responses to a
tone previously paired with a shock diminish, if the tone is repeatedly presented
without the shock, a process known as extinction. Since Parlor it has been
Line hypothesized that this extinction does not erase conditioning, but forms a new
(5) memory. Research has now demonstrated that destruction of the infralimbic
cortice blocks recall of fear extinction, indicating that it might store long-term
extinction memory. Infralimbic neurons recorded during fear conditioning and
extinction fire to the tone only when rats are recalling extinction on the
following day, and rats indicating the least fear responses also demonstrate the
(10) greatest increase in infralimbic tone responses. Conditioned tones paired with
brief electrical stimulation of infralimbic cortex elicit low fear responses in rats
that have not undergone extinction. Thus, stimulation resembling extinction-induced
infralimbic tone responses is able to simulate extinction memory.

According to the passage, behavioral psychologists studying the extinction process have discerned which of the following（）

A．The exact length of time required for a fear response to become extinct in a human subject
B．The effect of tone conditioning in comparison with other forms of stimuli
C．The possible effects of surgical operations on the infralimbic cortice
D．The potential of tone conditioning in treating undesirable fear responses
E．The limits of Pavlov's contribution to modern behavioral psychology