Dr. Persaud is curious as to whether children in a daycare center will share more after watching an educational video on sharing. Immediately before and after the video, she has several undergraduate research assistants code the sharing behavior of a group of 33 four-year-olds. Which of the following would you recommend to her to decrease the threat of instrumentation?

a. Using clear coding manuals

b. Using only one research assistant to code all the videos

c. Asking the same research assistants to code the same children at pretest and posttest

d. Establishing different coding manuals for pretest and posttest