Which of the Following Statements About Ramachandran Plots Is True $\phi$

Which of the following statements about Ramachandran plots is true?

A) They are good predictors of protein tertiary structure.
B) They are needed to determine the secondary structure of a protein.
C) They show equal distributions of $\phi$ and $\Psi$ angles for $\alpha$ -helical and $\beta$ -sheet containing proteins.
D) They show that $\beta$ -sheets and $\alpha$ -helices occupy different $\phi$ and $\Psi$ angles.