Idiom 'rolling budget'

[FangFang]

Business English Lesson, Advanced Level

ESL/EFL Test #155 "Budgeting", question 10

Many companies maintain a ......... budget, so that they are continually budgeting for this time next year.

(a) floating
(b) moving
(c) rolling
(d) shifting

Business English Lesson, Advanced Level

ESL/EFL Test #155 "Budgeting", answer 10

Many companies maintain a rolling budget, so that they are continually budgeting for this time next year.

Correct answer: (c) rolling
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Hi,
Why not choose "floating "budget? (though rolling budget is right, too, lol)

Thanks

[Mister Micawber]

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The idiom is 'rolling budget'; other than that, I can offer no rationalization.
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Native English teacher at Mister Micawber's