Task: Instance Description Understanding (ontology: Sciences/Computer_Science)

Which of the following definitions best describes the instance 'second'?

A. A time unit which is equal to 3600 seconds or 60 minutes.
B. A time unit which is approximately equal to the length of time of one of cycle of the moon's phases which in science is taken to be equal to 30 days.
C. A time unit which is equal to 7 days.
D. A time unit which is equal to the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Answer: A time unit which is equal to the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.