(1) Carla stated the ultimatum simply and clearly. (2) She had told Nicolas that unless he stopped bullying other kids in the classroom, she would go directly to their mom. (3) Nicolas fiddled with his thumbs. (4) Because he didn’t dare look at his sister as she towered over him. (5) Nicolas knew that Carla would follow through with her threat. (6) She had done so before, and he couldn’t risk being in trouble. (7) Nicolas had known he was in the wrong. (8) Nicolas couldn’t stand Carla judging him. (9) All he could do now was to look away and promise to never do it again.

What is the best way to rewrite sentences 3 and 4?

Nicolas fiddled with his thumbs, because he didn't dare look at his sister as she towered over him.

Nicolas fiddled with his thumbs because he didn't dare look at his sister as she towered over him.

Nicolas fiddled with his thumbs; because he didn't dare look at his sister as she towered over him.

Nicolas fiddled with his thumbs, because, he didn't dare look at his sister as she towered over him.