51. During gene transcription, E. coli RNA polymerase exists in both core enzyme and holoenzyme states. Which of the following statements about the transcription elongation phase is most correct? (A) The σ (sigma) factor must remain bound to the DNA during transcription elongation. (B) The core enzyme contains only a single protein subunit and can independently complete all transcription steps. (C) The σ factor dissociates after transcription initiation, and the core enzyme continues to participate and is responsible for transcription elongation. (D) The transcription elongation phase requires the assistance of TFIIH of RNA polymerase II.