Challenge 2: Collection or Environment Variable? — Possible Solution
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(a) baseUrl — ENVIRONMENT variable. This is the textbook case of a
value that genuinely DIFFERS per environment: localhost for dev, a
staging domain for staging, the real domain for production. Making it
a collection variable would defeat its whole purpose — switching
environments would no longer actually change where requests go.

(b) An API version path segment used identically in dev, staging, and
production — COLLECTION variable. Since this value is the SAME no
matter which environment is active, there's no reason to duplicate it
three times (once per environment) — defining it once at the
collection level means updating it in exactly one place if the API
version ever changes, rather than three.

(c) A per-environment API key — ENVIRONMENT variable. Like baseUrl,
this is expected to be genuinely different depending on which
environment is active (a dev key with limited scope vs. a real
production key) — and per this chapter's gotcha, ideally marked as
Postman's "secret" variable type given how sensitive a real API key
is.