การอบรม, ขององค์กร, คำบรรยาย, กระดานดำ, ชอล์ก, ครู, โรงเรียน, ศึกษา, การศึกษา, ออกแบบ, ความรู้
Public Domain
ภาพถ่ายปลอดค่าลิขสิทธิ์ที่เกี่ยวข้อง
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