What is one reason why the classification of protists in one kingdom is difficult

Biologist have developed Kingdom Protista in taxononmic grouping of living organisms but this kingdom includes organisms that are not particularly related. The organisms of this Kingdom exhibit features of other kingdoms like plant, animals and fungus. Some protists live as parasites, others as predators and some as producers like plants. For example, Euglena being a protists  have both animal and plant like characteristics, have chloroplasts like plants which allows them to gain energy from sunlight through photosynthesis and have a tail or flagella which facilitate them to swim, a characteristic common in animals. Therefore, classification of protists in one kingdom is difficult.  

the correct answer: the index of refraction of the second medium is lower i tried this persons answer and it was wrong this person is lying

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.