Answer:
B. The teachers have different amounts of teaching experience.
Step-by-step explanation:
The teachers having different amounts of teaching experience is the only problem that varies systematically along with the textbook version, so it is the only one that could be a confound. The Longer a teacher teaches a particular Course the more vast and learned he/she becomes and the more experience he/she becomes. He will know methods and things to do to ensure that the students understands better. Unlike a teacher that doesn't have much experience. So both of them handling the same course will definitely yield different results.