PCA (Prometheus Certified Associate) sample exam question with answer 556

Question:
Regarding the PromQL @ modifier, which of the following queries is NOT valid?

1. sum(up @ 1665982310)
2. up @ 1665982310
3. min_over_time(up[5m] @ 1665982310)
4. sum(up) @ 1665982310

Answer:
D - is the correct answer. The @ must be directly applied to a selector, not to the result of a function or aggregation. More at modifier
A, B, C - are incorrect as these are the valid use of the @ modifier, immediately following the selector