drop when In party or off party

Hello guys.
I want to ask you something about the drop pick up.
For example how it works if a raid is farmed by 1 cc (A) with 5 parties in it, 1 separate party(B) , and 1 person(C) ? Who will get the drop, A ,B or C ? Is there any scheme or calculations of the total dmg dealt or how does it works?

In this case, who give the last hit will have the loot priority for some first seconds after the boss's death. If only the CC were farming it, no matter.