Which of these is a receptor molecule? A. cell membrane with an integral molecule marked B. There is a specific recess in this molecule on the extracellular side. C. A molecule in the intracellular fluid, whose shape is identical to this recess, is marked A. D. Letter C marks a molecule connected to molecule B and is located in the cytoplasm. E. Letter D marks a molecule in the cytoplasm, near molecule C. F. Letter E marks a molecule in the cytoplasm located below molecule D.