I did not answer the door even though I knew...

Câu hỏi: I did not answer the door even though I knew it was my friend.

A. Unless I knew it was my friend, I would not answer the door.

B. I knew it was my friend, but I did not answer the door.

C. Only when I answered the door did I knew it was my friend.

D. I answered the door since I knew it was my friend.