I had to do my homework. I could not help my mother with the washing-up.

• A. Because I was busy doing my homework, I could not help my mother with the washing-up.
• B. It was impossible for me to do my homework although my mother helped me with the washing-up.
• C. I could not help my mother with the washing-up until I finished my homework.
• D. I could not do my homework because I had to help my mother with the washing-up.