My life is an open vault. vs My life is an open book.

[Mendy]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #546 "English Slang Idioms (149)", question 3

When asked if he minded recounting how he came upon the knowledge that allowed him to revolutionize the prosthetics field when he had no apparent medical training or schooling in biomechanics, he replied, "Not at all. My life is an open .........."

(a) account
(b) book
(c) vault
(d) door

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #546 "English Slang Idioms (149)", answer 3

When asked if he minded recounting how he came upon the knowledge that allowed him to revolutionize the prosthetics field when he had no apparent medical training or schooling in biomechanics, he replied, "Not at all. My life is an open book."

Correct answer: (b) book

Your answer was: incorrect
When asked if he minded recounting how he came upon the knowledge that allowed him to revolutionize the prosthetics field when he had no apparent medical training or schooling in biomechanics, he replied, "Not at all. My life is an open vault."
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why not open vault? aint there such idioms?

[Mister Micawber]

.
No, there is no such idiom. 'Ain't' is not a very educated word, either.
.
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Native English teacher at Mister Micawber's