Grammar. Subject and verb agreement.

Question

"Self-expression may be considered creative, but not all are worthy of being called art," Krysta Stafford wrote in her scathing review of the performance artist.

Subject, Not all is plural or singular?

I answered as are. but IXL said, correct answer is "is" instead of "are"

Answer 1

The sentence is poorly constructed and confusing. Self-expression is a singular noun and is the subject of the sentence. Not all is not correct in reference to a singular noun. A correct construction would be, "Self-expression may be considered creative, but not all forms are worthy of being called art."