To Find Out If Visual Imagery (I

To find out if visual imagery (i.e.,"picturing" things in one's mind) is more effective than verbal repetition in learning French vocabulary words,a French teacher 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 to just repeat the words over and over. She 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. Because all of the students used their given study guides and there was no reason to suspect other differences between the two groups of students,she concludes that the visual imagery study guide helped her students to learn their French vocabulary words more than the other study guide. Is the teacher's conclusion valid for her students?

A) No,because she used random assignment
B) No,because her experiment wasn't 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