Answer:
B. Sentence 2
Step-by-step explanation:
Sentence 2 is structured differently than the other sentences. Sentences 1, 3, and 4 use commas to separate the introductory phrase from the rest of the sentence. However, sentence 2 has two complete clauses which are separated by a semicolon. It still uses a comma after an introductory phrase (after the semicolon), but the semicolon sets it apart from the other sentences.