45. Retinoic acid, a metabolite of vitamin A, plays a key role in vertebrate development, differentiation, and organogenesis. Retinoic acid normally binds to the RAR receptor to initiate changes in gene expression. Researchers have determined that one type of RAR receptor is found at high levels in muscles, but is not found at all in neurons. How can you explain these findings? (A) Muscle cells, but not neurons, contain the gene for the RAR receptor. (B) The RAR receptor gene is expressed in muscle cells but not neurons. (C) Neurons can produce retinoic acid, so they do not need RARs for development. (D) RAR transcripts are hydrolyzed by lysosomes in neurons but not in muscle cells.