What is ‘minimum resolvable temperature’?

What is ‘minimum resolvable temperature’?
A . The detection threshold for passive infrared (PIR) sensors, calculated from the difference in temperature between an intruder, activity and the background.
B. The detection threshold for passive infrared (PIR) sensors, calculated from the difference in temperature between staged sensors
C. The detection threshold for passive infrared (PIR) sensors, calculated from the difference in temperature between an intruder and the site environment.
D. The detection threshold for passive infrared (PIR) sensors, calculated from the difference in temperature between an intruder and the background.

Answer: D