Notice (from your answer to Problem 15.8) that all of the isoprene units in farnesol and retinal are joined in a head-to-tail manner, but that they are not all arranged that way in squalene and b-carotene. Where does the head-to-tail arrangement break down in these two terpenes? (Hint: Count carbons to locate the terpene units.) What does this suggest about the way these two terpenes might be biosynthesized?