Tammy’s experiment compared the effectiveness of videotape and textbook presentation of a history lesson. Unexpectedly, her demeanor changed with each treatment condition. She appeared worried during the videotape condition because the audiovisual equipment periodically broke down. In contrast, she seemed relaxed in the textbook condition during which subjects read from a book. Which problem does this illustrate?a. confounding by a context variable
b. experimenter bias
c. history threat
d. selection threat