Units responded just before 9:00 AM for the apartment fire in the 100 block of O St., NW.  First arriving units found heavy fire issuing from the top floor of a two-story brick duplex apartment building.  Gaining fast entry, firefighters were able to engage an aggressive interior attack that extinguished the fire in minutes.   One firefighter suffered minor injuries. Investigators have determined that the errant use of candles started the fire, which caused $40,000 worth of damage.  There were no civilian injuries, and one person was displaced.