The Following Is a Description of an Enzyme Called Superoxide

The following is a description of an enzyme called superoxide dismutase.Refer to this paragraph to answer the following question. Superoxide dismutase is an enzyme that detoxifies a free radical called superoxide.This radical is responsible for causing injury during reperfusion after a heart attack, and is also thought to play a role in Parkinson's Disease.^a Depending on what organism the protein is found in, it occurs either as a dimer or tetramer of identical subunits.^b The subunits are mostly α-helical with some β-sheet near the C-terminus.^c A metal binding site for manganese, iron, nickel or copper/zinc is composed of residues from both the N-terminus and the C-terminus.^d These residues are: His-26, His-81, Asp-167, His-171.^e
Which statement refers to quaternary structure?

A) statement a
B) statement b
C) statement c
D) statement d
E) statement e