How Did Researchers Surmise That Being in Lower Level Classes

How did researchers surmise that being in lower level classes can reduce student performance?

A) The effect was proved by experiments that randomly assigned students to lower level classes and measuring the differences.
B) They measured performance enhancements from being in gifted classes and compared that with performance from lower level classes.
C) Interviews with family members and students were compared with standardized test results from students in both types of classes.
D) A longitudinal study was done to compare lifetime outcomes in terms of career advancement and educational attainment.