The Difference Between 1 Second of Sidereal Time and 1

The difference between 1 second of sidereal time and 1 second of solar time is:

A) very small but variable because of the variable motion of Earth in its orbit.
B) 0, since they are defined to be equal.
C) a very small but finite and fixed interval of time.
D) very large, because 1 sidereal second measures a fraction of 1 year while 1 solar second measures the same fraction of 1 solar day.