Which statement is TRUE regarding the gamma ( $\gamma$ ) subunit of ATP synthase?

A) It contains mostly $\alpha$ -helical regular secondary structure.
B) It exists as a dimer in the F₁ portion of ATP synthase.
C) It prevents the F₁ portion from rotating relative to the F_O portion.
D) It interacts with the a and b subunits of the F_O portion.
E) It binds protons as part of the proton translocation process.