Which of these is NOT a factor that may lead to different conclusions from two different studies investigating the same thing (eg., the relationship, if any, between caffeinated coffee and memory)?

A. using a control group
B. different sample sizes
C. different amounts of caffeine
D. different ways of evaluating memory (eg., long-term vs. short-term memory)
E. different types of participants (eg., participants of different ages or professions)