Which statement is true concerning the structure of proteins?

A. The primary structure is the sequence of amino acids.

B. Alpha helices and beta sheets are examples of secondary structure.

C. Side chains (R-groups) of amino acids can be hydrophilic or hydrophobic.

D. Proteins made of two or more polypeptide chains have quaternary structure.

E. All statements are true.

This is a good summary of important features to know about protein structure.