Lucy Examined Relationships Between Middle-School Students' Self-Esteem and Their Performance

Lucy examined relationships between middle-school students' self-esteem and their performance in Mathematics.Her data analysis indicated that students with higher self-esteem perform better than those with lower self-esteem.Her investigation further revealed that students with higher self-esteem are more willing to invest effort in solving Math problems.In this case,the amount of effort is:

A) an independent variable.
B) a dependent variable.
C) a mediating variable.
D) a moderating variable.