Because Students at Private High Schools Get Better Test Scores,on

Multiple Choice

Because students at private high schools get better test scores,on average,than those in public schools,a researcher concludes that the private high schools provide a better education.What is a major weakness of this study,from a scientific standpoint?​

A) Students were not randomly assigned to groups.
B) ​Public high schools tend to be larger than private high schools.
C) ​Private high schools are more expensive than public high schools.
D) ​Many students prefer public schools, because they have friends there.