Answer:
Subject
Explanation:
Your sentence always needs a subject if not then your sentence would make no sense because you wouldn't be talking about a certain thing.
<span>It was combined with the theory of continental drift to form one unified explanation. </span>
Answer: I think the second line is correct it you put a apostrophe in the first and before Simpson.
Explanation: The third line is also correct it is a I;I compound sentence. But the first line has some mistakes.