C Duct is the name of the reverse cowls. It seems that the panel in the photo is the one that cover the c-ducts latches, below engine. Must problably blowed out as a result of explosion.
(In this video, the one showed opened at 3:17
)
Oxygen masks deploy automatically when airplane is above 10,000 feet more or less. So for them to be deployed so low, pilots should use a dedicated switch which deploys them no matter how highh the plane is.
Deploying the mask, pilots must probable avoided pass affected by smoke. It was a very good decison.