If B arrived before C, B had the right of way----stop sign or traffic light.
If C arrived before B, C had the right of way----stop sign only.
C must yield to the on-coming traffice when making a left turn---stop sight or traffic light
我個人認為說他錯了。因為您早到。 但是, CA DMV 說您錯了。 here is the quote
"When you turn left, give the rightof- way to all vehicles approaching that are close enough to be dangerous. Also, look for motorcyclists, bicyclists, and pedestrians. "
Source: California Driver Handbook - Laws and Rules of the Road http://www.dmv.ca.gov/pubs/hdbk/right_of_way.htm