Cash vs in cash

[Greta]

Test No. ESL Test "Letter of Complaint", question 4

In January this year I bought a washing machine from you for which I paid .........

(a) cash
(b) money
(c) cheque
(d) pounds

Test No. ESL Answer "Letter of Complaint", answer 4

In January this year I bought a washing machine from you for which I paid cash

Correct answer: (a) cash

Your answer was: incorrect
In January this year I bought a washing machine from you for which I paid money
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Hi,

Why we use cash without any preposition in ihe following sentence?

I've chosen money because people usually say "pay in cash". I heard some use "with" or "by".

Thanks
Greta

[Mister Micawber]

.
Here, the preposition in is optional. We pay (in) cash rather than pay e.g. by credit card. On the other hand, money does not work because pay money is redundant and an unnatural collocation-- paying includes of necessity money.
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Native English teacher at Mister Micawber's