单项选择题

Art is considered by many people to be little more than a decorative means of giving pleasure. This is not always the case, however, at times, art may be seen to have a purely functional side as well. Such could be said of the sand paintings of the Navaho Indians of the American Southwest; these have a medicinal as well as an artistic purpose.
According to Navaho traditions, one who suffers from either a mental or physical illness has in some way disturbed or come in contact with the supernatural--perhaps a certain animal, a ghost or the dead. To counteract this evil contact, one of his relatives will employ a medicine man called a "singer" to perform a healing ceremony which will attract a powerful supernatural being. During the ceremony, which may last from 2 to 9 days, the "singer" will produce a sand-painting on the floor of the Navaho Hogan. On the last day of the ceremony, the patient will sit on this sand-painting and the "singer" will rub the ailing parts of the patient’s body with sand from a specific figure in the sand-painting. In this way the patent absorbs the power of that particular supernatural being and becomes strong like it. After the ceremony, the sand-painting is then destroyed and disposed of, so its power will not harm anyone.
The art of sand-painting is handed dawn from old "singers" to their students. The materials used are easily found in the areas the Navaho inhabit; brown, red, yellow and white sandstone, which is pulverized by being crushed between 2 stones much as com is ground into flour. The "singer" holds a small amount of this sand in his hand and lets it flow between his thumb and forefingers onto a clean, flat surface on the floor. With a steady hand end great patience, he is thus able to create designs of stylized people, snakes and other creatures that have power in the Navaho belief system, the traditional Navaho does not allow reproduction of sand-paintings, since he believes the supernatural powers that taught him the craft have forbidden this; however, such reproductions can in fact be purchased today in tourist shops in Arizona and New Mexico. These are done by either Navaho Indians or by other people who wish to preserve this craft.
The "singer" rubs sand on the patient because ______.