Fall into debt

Test No. [color=blue]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
© owing
(d) owes

Test No. [color=blue]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: [color=red]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?

To [color=blue]fall into debt is a fixed phrase.[YSaerTTEW443543]

TOEIC listening, photographs: The laptop on the round table[YSaerTTEW443543]

I should’ve guessed it. mercy…