21. A major difference in the mechanism of action between steroid and peptide hormones is that
(A) steroid hormones mainly affect the synthesis of proteins, whereas peptide hormones mainly affect the activity ofproteins already in the cell.
(B) target cells react more rapidly to steroid hormones than they do to peptide hormones.
(C) steroid hormones enter the nucleus, whereas peptide hormones stay in the cytoplasm.
(D) steroid hormones bind to a receptor protein, whereas peptide hormones bind to G protein.
(E) steroid hormones affect metabolism, whereas peptide hormones affect membrane permeability.