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 toinitiate changes in gene expression. Researchers have determined that one type of RARreceptor is found at high levels in muscles, but is not found at all in neurons. How can youexplain 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.