Why does the author include a description of how the "doorways and windows" of Machu Picchu were constructed? A. To indicate that the combined skeletons and skins of the stone buildings of Machu Picchu were similar to igloos and adobe structures B. To indicate the different kinds of stones that had to be cut to build Machu Picchu C. To provide an illustration of the kind of construction that was required before arches were invented D. To explain how ancient builders reduced the amount of time necessary to construct buildings from stone