Aresearcher is interested in examining students' understanding and recall of texts. she gives students a series of texts to read (either essays or stories matched on critical variables such as length and grade level and presented randomly) and trains assistants to score and count up the exact number of correct "idea units" (or unique phrases) students recall from each text. the researcher also interviews students after they have read each text to examine their thoughts on why each was easy or difficult to understand. the researcher records students' responses and trains assistants to examine students' answers for different themes.
the researcher's study would fit best into the category of:

a. a mixed-methods study.
b. a quasi-experimental study.
c. a qualitative study.
d. a quantitative study.