Why are phi and psi bond angles significant in the context of beta sheets?

• A. They only affect the formation of alpha helices.
• B. They are irrelevant to the structure of beta sheets.
• C. They are used to predict the primary sequence of proteins.
• D. They determine the specific folding pattern and stability of beta sheets, represented in the upper left quadrant of a Ramachandran plot.

Answer: (D)