Sometimes we can detect the presence of very weak stimuli and sometimes we can not

This is due to the

a. special properties of the stimulus we are attempting to detect.
b. inherent variability in the activity of our nervous systems.
c. random defects in our receptor cells.
d. psychophysical properties of stimulus detection problems.
e. psychophysical properties of environmental conditions.

b. inherent variability in the activity of our nervous systems.