Unlike Somatic Hypermutation, Class Switching Occurs in Discrete Sequence Regions

Unlike somatic hypermutation, class switching occurs in discrete sequence regions upstream of the immunoglobulin heavy chain coding sequences (called switch regions) . One key element in directing the enzyme AID to a specific switch region is the opening of the DNA duplex combined with polymerase stalling during active transcription in that region. A second key feature of directing AID to a specific switch region is:

A) The processed RNA from the switch region guides AID to this site in the DNA
B) The binding of DNA-PK's to the switch region sequence in the DNA
C) The presence of double-stranded breaks in the DNA in this region
D) The binding of AID to the RNA polymerase that is transcribing the switch region
E) The predominance of G:C base pairs in the switch sequence