Answer: There was thunder and there was a flash of lightning.
Explanation: The clause does not express a complete thought and cannot stand on its own as a sentence. ( there was thunder).
( there was a flash of lightning) is the dependent clause. It contains the subject he and the verb injured. since it says there was the thunder that would be the clause so since there was thunder it would make it lightning so that would the dependent clause because the thunder caused the lightning.
Hope this helps, good luck.