At What Point Does the Sigma (S) Subunit of RNA

At what point does the sigma (s) subunit of RNA polymerase released from the core enzyme?

A) Prior to the incorporation of any nucleotides into an RNA strand.
B) After transcription begins and about 10 nucleotides have been added to the RNA chain.
C) Just prior to chain termination.
D) Never; it is an intrinsic part of the core enzyme.