Taking natural objects such as rocks, bones, clouds and flowers for subject matter, Georgia O'Keeffe reduced them to their st form, often by employing a close-up view or some other unusual vantage point. With such techniques, including the use of thin paint and clear colors to emphasize a feeling of mystical silence and space, she achieved an abstract simplicity in her paintings. O'Keeffe spent a summer in New Mexico in 1929 and the bleak landscape and broad skies of the desert so appealed to her that she later settled there permanently. Cows' kulls and other bare bones found in the desert were frequent motifs in her paintings. Other common subjects included flowers, the sky, and the horizon lines of the desert. After O'Keeffe's three-month trip around the world by plane in 1959, the sky "paved with clouds" as seen from an airplane also became one of her favorite motifs and the subject of her largest work, a 24-foot mural that she began in 1966.With what subject is the passage mainly concerned? ( )