Mark conducts a study for his research methods class to determine if consuming caffeine causes people to perform better on cognitive tasks. In his study, he gives half of his participants a glass of water and half of his participants a glass of cola, and then has them attempt to solve 15 math problems. He finds that people who drink caffeine beforehand solve more math problems than those who drink water beforehand.
Mark decides to conduct a variation of the original study in which he measures participants' ability to solve verbal analogies as the cognitive task instead of measuring their ability to solve math problems. He finds that people who drink caffeine beforehand solve more verbal analogies than those who drink water beforehand, suggesting caffeine consumption causes improved cognitive performance. This study is known as a:
a. Statistical replication
b. Replication plus-extension
c. Conceptual replication
d. Direct replication