An instructor randomly assigns two sections of his course to an experimental condition (new teaching method) or control condition (regular teaching method). Students in the 9:30 section receive the experimental treatment and students in the 1:30 section participate in the control condition. At the end of the semester the instructor’s tests reveal that students in the 9:30 section had significantly higher test scores than students in the 1:30 section. Based on this summary, we can state that
A. the results are uninterpretable because of intact groups.
B. the findings have external validity across two times of day.
C. the experiment has internal validity.
D. all of these
