A word group that begins with a subordinating word, relative pronoun, or transitional word or phrase, but does not include an independent clause, is considered a D. sentence fragment. Sentence fragment never contains independent clause, but it usually has either a dependent clause or a phrase. Sentence fragments also miss a subject or even a verb but they begin from subordinators like 'after, 'before', 'when' (etc.) which make it look like a complete sentence, but it makes no sense if it's separated from a dependent clause.
