What does this idiom mean: water under the bridge

[ROselyn]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #408 "Water Idioms", question 5

He's decided to make a fresh start and regards all his old problems as water under the bridge -- means: ..........

(a) not worth looking at
(b) not worth worrying about
(c) not worth talking about
(d) not worth dreaming about

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #408 "Water Idioms", answer 5

He's decided to make a fresh start and regards all his old problems as water under the bridge -- means: not worth worrying about.

Correct answer: (b) not worth worrying about
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what does this idiom mean?

ROselyn

[Mister Micawber]

.
'It is water under the bridge' means that it has happened, for better or worse, and cannot be changed, so it might as well not be worried about any more.

For instance, if I fail a job interview, I will not be hired by that company, so there is no point in my worrying about that interview; I should just start job-hunting again.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber