บทคัดย่อ, ดวงอาทิตย์, ซ่าน, ดาว, รูปร่าง, ทางเรขาคณิต
Public Domain
ภาพถ่ายปลอดค่าลิขสิทธิ์ที่เกี่ยวข้อง
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