Answer:
The answer is: D. I rang the doorbell, but no one came to the door. I was sure this was the right address. While I waited, I saw a curtain move.
Explanation:
It is generally much better to have short sentences than a long involved sentence. The long sentence in the example is pretty easy to understand, but selection "D" makes the meaning clearer and easier to understand. Separate sentences cuts down on trying to figure out multiple meanings.