Answer:
Molly left the house early, so she got to the party on time.
Step-by-step explanation:
Compound sentences contain at least two independent clauses which have relevant ideas. They are independent because they can still stand on their own. However, they can be joined by a comma and a coordinating conjunction, or by semicolon.
Sentences two and four are simple sentences because they consist of a single clause, while sentence three is a complex sentence and possesses an independent clause (The students did well on the test) and a dependent clause (because they studied for many hours).