How Can Two Different Studies Investigating the Same Thing (E

How can two different studies investigating the same thing (e.g. ,the relationship,if any,between caffeinated coffee and memory) come to different conclusions?

A) They may have had different sample sizes.
B) They may have used different types of participants (e.g. ,participants of different ages or professions) .
C) They may have used different amounts of caffeine.
D) They may have evaluated memory differently (e.g. ,long-term vs.short-term memory) .
E) All of the answers are correct.