'as a joke' vs 'as joking'

[Arno]

English Idioms and Expressions, Elementary Level

ESL/EFL Test #4 "For Good", question 6

I know it didn't look like that but the truth is they only did it for fun.

(a) as a joke
(b) as humour
(c) as funny
(d) as joking

English Idioms and Expressions, Elementary Level

ESL/EFL Test #4 "For Good", answer 6

I know it didn't look like that but the truth is they only did it as a joke.

Correct answer: (a) as a joke
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what the the different between the choices especially a joke and joking

Arno

[Diverhank]

Since we are talking about Idioms and Expressions..."to do something as a joke" is an expression. "As humor, as funny and as joking" simply do not look and sound right as normal English.

[Wms]

So diverhank, we can't use " As humar, as funny and as joking " while speaking about making fun!