idiom 'a smoking gun'

[Menke]

English Language Tests, Intermediate level

ESL/EFL Test #853 "'Guns Etc:' Common Usages (2)", question 7

The missing ledger was the ......... gun that would ensure a conviction. The FBI personnel were frantically searching for the ledger that would prove the accountant had been embezzling money for years.

(a) shot
(b) smoking
(c) big
(d) firing

English Language Tests, Intermediate level

ESL/EFL Test #853 "'Guns Etc:' Common Usages (2)", answer 7

The missing ledger was the smoking gun that would ensure a conviction. The FBI personnel were frantically searching for the ledger that would prove the accountant had been embezzling money for years.

Correct answer: (b) smoking
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Would you use smoking gun, firing gun or smoking gun for a "ledger that was the ... gun that would ensure a conviction."

Menke

[Beeesneees]

'a smoking gun' is an idiom. This is a very special phrase that means indisputable evidence. In other words, a smoking gun is a piece of evidence that proves that something is true or that someone is responsible for a crime.

No other term including the word 'gun' means the same.
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"Inside every old person is a young person wondering what happened."
Terry Pratchett