-Thank you for helping me with my washing.-() A.Not at all B.It doesn't matter

选A,
Not at all 别客气
For example：
A：Thank you very much.多谢你了.
B：Not at all.不客气.
It doesn't matter.则表示没关系,是回答道歉的.
A：I apologize for leaving you alone.很抱歉把你一个人留下.
B：It doesn't matter.没关系