"At once"

Hi
in Greg Mat videos, at once shows contrast, right? I can’t find it in the contrast list, but I also don’t know which exact lecture he said it in, which is why I just thought I would ask the larger crowd.
”At once” in sentence equivalence does show a contrast, right? For example, at once ____ and ___. So something like at once hot or cold, showing a contrast, right? It won’t be at once hot and warm.

So now, I came across this question - (3rd party)
Because the historian’s account was at once (i) __ and (ii) __, it succeeded in portraying political upheavals without reducing them to simple moral narratives; her conclusions, though complex, possessed an unexpected (iii) ___

A. impartial. B. polemical. C. speculative
D. meticulous. E. careless. F. anecdotal
G. clarity H. cynicism I. novelty

here I chose AFG, but the answer key says ADG>

Any help is really appreciated.

I really don’t recommend 3rd party verbal.

At once is usually a contrast though.