17.4. References http://www.ck12.org
- Christopher Auyeung. CK-12 Foundation. CC BY-NC 3.0
- Subway: Metropolitan Transportation Authority of the State of New York; Light switch: wilB (Flickr:idea
ablaze). Subway: http://www.flickr.com/photos/mtaphotos/8879516489/; Light switch: http://www.flickr.
com/photos/ideaablaze/4932631175/. CC BY 2.0