A subordinating adverb include option C: Even so, when I was invited to attend an important author event on the East Coast, a friend took me in hand.
What do you mean by Subordinating Adverb?
Subordinate adverb can be punctuate through comma before the subordinating adverb phrase or clause.
Moreover, subordinate adverb clauses are defined as the clause that are dependent clauses created by more word combinations that are act like an adverb.
The subordinate clauses are begins with subordinating conjunctions like after and since etc. In a third option adverb punctuate correctly because after comma it starts with "when"
Therefore, correct option is C.
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