What is the subordinate clause in this sentence?

as i was walking, i stumbled upon a beautiful seashell.

a.
as i was walking

b.
i stumbled upon

c.
a beautiful seashell

The subordinate clause in this sentence is A As I was walking

A subordinate clause begins with a conjunction or a relative pronoun and it doesn't have all the necessary information no convey an idea, Option A best represents this kind of clause, Options B and C are part of the main clause so they can not be considered as two different clauses.

third person

explanation:

note the usage of names and pronouns, "james" & "the old man". this alone can confirm it as third person, as first person uses "i" and the like, while second person uses "you" and the like.

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choice 2 i like that prompt better.