Answer:
C
Explanation:
The sentence which is correctly punctuated is the last one. First of all it's not necessary to have a comma before the "since", so the second one is discarded. Secondly as the first sentence doesnt have the final stop, it's also discarded because without it, it cannot be considered a complete sentence. Finally, the third sentence is the only one which has the full stop at the final of it and it also doesn't have a comma in the middle of it, so it's the correct answer.
