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Introducing your story like this is no problem, however if C is normally unrelated to the story you may consider the first chapter not being a de facto chapter and call it an introduction/prologue ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/7827 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Introducing your story like this is no problem, however if C is normally unrelated to the story you may consider the first chapter not being a de facto chapter and call it an introduction/prologue instead. It is a common characteristic of an introduction to introduce the plot via a side character. Normally this is a very short chapter comprising of one short scene. If your chapter falls into this description you may want to consider making it an Introduction/prologue.