Which of the following statements concerning ribosomes are true?

• A. Several ribosomes are often attached to and translating the same mRNA.
• B. Ribosomes join amino acids to form a polypeptide.
• C. Ribosomes have a binding site for mRNA and three binding site for tRNA molecules.
• D. No protein synthesis within a cell would occur without ribosomes.
• E. All of the above statements are true.