What is A

Sandy painted a picture.
Jon looked at it.
"I've never seen blue mountains."
Jon said.
Sandy thought "Jon doesn't like my picture. He thinks it is strange.
If he doesn't like my mountains he probably doesn’t like my sky
If Jon doesn't like my picture, I am not a good painter.
If I am not a good painter, I am not a good artist. If I am not good at art,
I am not good at anything. I am not much good at all
I am worthless. Nobody likes me. Sandy looked sad.
Sandy felt very sad. She did not paint any more
She just sat and looked sad the rest of the day.

When her teacher asked why she looked so sad, she said,
"John didn't like my picture."
Do you think that is a true answer.
Sandy thought she was telling the truth.
What really made Sandy feel so sad.

What is A in this story.
What is B
What is C

Finish this sentence:

It was ____________________________________________________________________
about _____________________________________________________________________

that "made" Sandy feel ___________________________________________________,