Knockout Mice That Lack Either the Gene for the Precursor

Knockout mice that lack either the gene for the precursor for substance P or the gene for the substance P receptor exhibit

A) total insensitivity to pain stimuli.
B) insensitivity to mildly painful stimuli but normal responses to intensely painful stimuli.
C) insensitivity to intensely painful stimuli but normal responses to mildly painful stimuli.
D) exaggerated responses to mildly painful stimuli.