TOEFL Listening: ETS-TOEFL听力机经 - LW97S84B9J1SU49EP$

According to the professor, what concern did the staircase procedure in the experiment address? A. The difficulty of the meta-cognitive task for some subjects. B. The improvement in subjects' abilities over the course of the experiment. C. The small amount of data collected from the experiment. D. The difference in perceptual abilities of the subjects.