What does 'set in stone' mean?

[Stett]

Business English Lesson, Advanced Level

ESL/EFL Test #188 "Dotcom Retail Stores", question 2

In the traditional realm of business, you open a retail store first and then launch a website, but since the dotcom revolution, many surviving dotcoms that skipped that first step are realizing the benefits of opening a ......... location.

(a) brick-and-mortar
(b) plaster-of-paris
(c) rock-solid
(d) set-in-stone

Business English Lesson, Advanced Level

ESL/EFL Test #188 "Dotcom Retail Stores", answer 2

In the traditional realm of business, you open a retail store first and then launch a website, but since the dotcom revolution, many surviving dotcoms that skipped that first step are realizing the benefits of opening a brick-and-mortar location.

Correct answer: (a) brick-and-mortar
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set-in-stone

[Mister Micawber]

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Set in stone = unchangeable.
.
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Native English teacher at Mister Micawber's