A should be the right answer
a, a, b, b, a, b, b, b, b, b, a, b, b, a, b, a, b, a, a, a. Hope I helped! All of them should e correct too! :)
Answer:
I know its not an idiom, irony and I dont think its the last one so mabey a oxymoron?? Im sorry
Explanation:
SEQUENCING is arranging things in order so they can be numbered or related in a connected series.