2019 Appeal Exam Question

[youngump]

Bottom of the 7th inning, score is tied with 1 out and R1 is on 3B, R2 is on 2B and R3 is on 1B. B5 hits a fly ball over F9’s head scoring R1 from
3B. Thinking the game is over, R2 and R3 do not advance and touch their respective bases and B5 does not touch 1B. Instead they join in
celebrating with team members at home plate. F1 standing near the pitching plate makes a verbal appeal to the base umpire that B5 did not
touch 1B and the base umpire calls B5 out. Then F4 appeals that R3 did not touch 2B and the base umpire calls R3 out. The correct ruling is:

I'm not sure if this question is going after when the ball becomes dead at the end of the game or not. (So as to whether we can accept a dead ball appeal.) And that's maybe an interesting discussion. But let's just say that the ball becomes dead. If so, I'm a little torn about calling R3 out for missing second. She didn't pass it so she's not assumed to have touched it and she's not forced there if you accept the appeal at first. Abandonment doesn't apply as the runners are still in live ball territory.

The correct answer on the test (eliminating the three that must necessarily be wrong) suggests that this is a proper dead ball appeal, but I'm lost on it. Thoughts?