This protein folds into a doughnut shape that can encircle DNA. It can load on the DNA only when the DNA is broken in both strands, so that the DNA can thread through the hole in the protein. Which of the following proteins do you think matches this description?

A) PCNA, the sliding clamp for DNA polymerases at the replication forks
B) Ku, the protein that recognizes DNA ends and can initiate nonhomologous end joining
C) MCM, the helicase critical for the initiation and elongation of replication
D) Topoisomerase II, which can create or relax superhelical tension in DNA
E) RecA/Rad51, which carries out strand invasion in homologous recombination