Exercise 1: Why an Action, Not a Filter, for a Price Change — Possible Solution
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WHAT DETERMINES ACTION VS. FILTER
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Per this chapter's own recap table, the distinction isn't about
whether a value changes - it's about the hook's own shape: a filter
exists to modify a specific value and must return it, while an action
exists to "do something at this point" with no return value expected.

WHY woocommerce_before_calculate_totals IS AN ACTION
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Per this chapter, this hook acts on the whole $cart object passed
into it - looping through every cart item and calling set_price()
directly on each product object inside the cart, rather than
receiving a single price value, modifying it, and returning the
modified value. There's nothing to "return" here; the function
mutates the cart's own item objects directly, which is exactly the
action pattern rather than the filter pattern - the chapter states
this directly: "an action, despite modifying a price, because it acts
on the whole cart object rather than returning a single filtered
value."

WHY THIS WORKS AS AN ANSWER
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It correctly identifies that action-vs-filter depends on the hook's
own shape (mutate-in-place vs. modify-and-return), not simply on
whether the end result changes a price, and correctly applies that
distinction to explain why this specific hook is an action.