LOD on 4th down

[MTUMP]

Situation: 4th and 5 for A at B's 29.5 yard line with 15 seconds left in 4th Quarter and A behind by 4 points. QB A7 roles out and runs for first down, but just before he's down he throws an incomplete IFP into the endzone at B's: A) 18 yard line, B) 22 yard line, or C) 19.5 yard line. There are 5 seconds left on the clock.

Enforcement? I think the options for A and B are fairly straight forward, but am interested in hearing about any special mechanics to help enforce the situation in C.