TPO41-Navajo Art Questions

阅读的第四题C选项：C. Navajo rituals take place indoors.对应的文章中的一句话为：The largest of these paintings may be up to 5.5 meters in diameter and cover the entire floor of a room. Working from the inside of the design outward, the Navajo artist and his assistants will sift the black, white, bluish-gray, orange, and red materials through their fingers to create the finely detailed imagery.为什么就可以理解为发生在室内？