Fall into debt

[Rich7]

Test No. incompl/advan-19 "Money Matters (2)", question 2

The company fell heavily into ......... when it took on too many customers and offered them all cheap warranties.

(a) debt
(b) debts
(c) owing
(d) owes

Test No. incompl/advan-19 "Money Matters (2)", answer 2

The company fell heavily into debt when it took on too many customers and offered them all cheap warranties.

Correct answer: (a) debt

Your answer was: incorrect
The company fell heavily into debts when it took on too many customers and offered them all cheap warranties.
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why the use of the singular in this case?

[Torsten]

To fall into debt is a fixed phrase.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary

[Rich7]

I should've guessed it. mercy...