First vs firstly vs at first

[Duc]

Hello,
please help me with the sentence below:
Everyone congratulate July on her suggestion,but actually I thought of it........
A.first B.at first C.at the beginning D.firstly
The answer is A but i think B and D are possible. Am I right ?
_________________
Don't see your shade think you are great

[Mister Micawber]

.
No. 'At first' is used as a temporal contrast ('at first I couldn't do it, but now I can'). 'Firstly' (a useless adverb meaning 'first') is used as a sentence adverb only.
.
_________________
Native English teacher at Mister Micawber's