Which variable could be considered a confounding variable in the research study described below? This study was designed to measure the differences in scores on a state-administered math achievement test between fourth-grade students who participated in an after-school reading enrichment program versus those who participated in an after-school math enrichment program? Group of answer choices a) Grade level of the students b) Time of day of the enrichment program c) Students' math scores in third grade d) Students' scores on the test