A French Teacher Reads an Article About How Visual Imagery

A French teacher reads an article about how visual imagery (i.e., "picturing" things in one's mind) can be used to help students learn French vocabulary words.To find out if visual imagery is more effective than verbal repetition in learning vocabulary words, she develops two different study guides for her students-one that tells students how to use visual imagery to learn French words, and one that tells them just to repeat the words over and over again-and randomly distributes the two study guides to her students.Over the next few weeks, the teacher finds that students using visual imagery study guides achieve higher average quiz scores.She concludes that the study guides describing the visual imagery technique help her students learn their French vocabulary words.Is the teacher's conclusion valid?

A) No, because she used random assignment.
B) No, because her experiment was not conducted in a laboratory.
C) Yes, because her students probably all had similar IQ scores.
D) Yes, because she was able to manipulate a variable in the environment.