If you read each of these, or speak them, there is only one that suits the sentence correctly.
Go with the third one. Couldn't seems to fit pretty well there.
Answer:
1: Traffic was moving at a snail's pace.
Explanation:
An Idiom is like a phrase:
It's raining cats and dogs out there.
"Raining cats and dogs" is the idiom, as with this answer, it's, "moving at a snail's pace"
Answer:
a Janus mask is a mask with two faces used by actors to allow them to play two characters, the term "Janus mask" comes from the Greek mythical character "Janus" who was the two faced god of choices.
Explanation:
hope this helps :)
