an image from the US government EPA "Sunwise" program. I converted it from PDF format, and removed all the "sunwise" logos. the source link is here - http://www.epa.gov/sunwise/doc/mission_story.pdf
孩子們在黑板前
blackboard、chalkboard、child、classroom、epa、epa sunwise、hat、kids、People、people - cartoon - cute
黑板、黑板、兒童、課堂、EPA、epa sunwise、有、孩子、人、人 - 卡通 - 可愛
資料來源：openclipart ( https://openclipart.org/ )