Busy doing sth. vs. active in doing sth

Test No. [color=blue]incompl/inter-26 “The Ants and the Grasshopper”, question 2

What you could hear was the ants who were drying grain.

(a) active
(b) doing
(c) busy
(d) business

Test No. [color=blue]incompl/inter-26 “The Ants and the Grasshopper”, answer 2

What you could hear was the ants who were busy drying grain.

Correct answer: (c) busy

Your answer was: [color=red]incorrect
What you could hear was the ants who were active drying grain.
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what about active here?

Here you would have to say active IN