บทคัดย่อ, คลื่น, ทิศทาง, คอมพิวเตอร์กราฟฟิค, กราฟิก, ลูกศร, ภาพเงา
Public Domain
ภาพถ่ายปลอดค่าลิขสิทธิ์ที่เกี่ยวข้อง
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