Which of the Following Statements Regarding Cooperative Binding Is True

Which of the following statements regarding cooperative binding is true?

A) Two or more proteins aid the binding of a ligand.
B) The binding of one ligand aids the binding of a second ligand
C) The folding of part of a protein aids in the folding of the remainder of the protein
D) All proteins engage in cooperative binding
E) The binding of a cofactor to a protein aids in the binding of a ligand.