บทคัดย่อ, ศิลปะ, พื้นหลัง, สีน้ำเงิน, ศูนย์, สี, ใต้น้ำ, ทางเรขาคณิต, เรขาคณิต, กราฟิก, เครื่องหมาย
Public Domain
ภาพถ่ายปลอดค่าลิขสิทธิ์ที่เกี่ยวข้อง
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