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
© 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: © busy

Your answer was: [color=red]incorrect
What you could hear was the ants who were active drying grain.
[size=200]_________________________[/size]

what about active here?

Here you would have to say active IN