anything

[jiang]

Dear teachers,

Please read the following:

"It's very kind of you to help me." "Oh, that's all rihgt. _________, I enjoy it."
a. It was nothing b. It wasn't anything.
The key is "a". Could you please explain why "b" isn't correct?

Looking forward to hearing from you.

Thank you in advance.

Jiang

[Tdol]

It's just a standard expression for this situation. As everybody says it, the other answer sounds strange.