Busy doing sth. vs. active in doing sth

[Rich7]

Test No. 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. 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: incorrect
What you could hear was the ants who were active drying grain.
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what about active here?

[Alan]

Here you would have to say active IN
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