Because Students Who Took a Calculus Course Do Better in Biology

Multiple Choice

Because students who took a calculus course do better in biology than students who did not take calculus,a researcher concludes that understanding calculus is helpful for understanding biology.What is wrong with this study?​

A) Calculus is a more difficult subject than biology.
B) ​Biology is a more difficult subject than calculus.
C) ​The research had too many independent variables.
D) ​Students were not randomly assigned to groups.