Why is it difficult to classify protists? Select three options.

There are many varying characteristics and exceptions to each type of protist.
They have been previously categorized based on what they are not.
They are all eukaryotes composed of one or more cells that contain nuclei.
There are no exceptions to the characteristics of each type of protist.
They are microscopic and therefore too small to study accurately.
Recent studies show that protists have not descended from one common ancestor.

Please help me, I have a bad grade in science so far : (