Can the letter "a" be a preposition?

[Karl]

English Grammar Tests, Elementary Level

ESL/EFL Test #48 "Common Prepositions", question 10

I am ......... work right now.

(a) with
(b) at
(c) of
(d) on

English Grammar Tests, Elementary Level

ESL/EFL Test #48 "Common Prepositions", answer 10

I am at work right now.

Correct answer: (b) at

Your answer was: correct
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Can the letter a be a preposition?

Thanks
Karl

[Alan]

Hi Karl,

No it can't.

Alan
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English as a Second Language
You can read my ESL story Indirect Speech

[Dongocan]

hi, Alan , why we use the prep " at " instead of "on" here!? please,tell me more!!!thx in advance