In a Test of the Impact of Delay on Recall

In a test of the impact of delay on recall, a researcher has two groups study a list of words. For the immediate recall condition, the researcher has the subjects recall the words immediately after the study period has ended. For the delay recall condition, the researcher has the subjects recall the words three minutes after the study period has ended. The researcher found that there was not a significant difference in the number of words recalled between the two conditions. The researcher decides to conduct the study again, this time using fifteen minutes as the delay between studying and recalling the list. The researcher is trying to optimize t by

A) increasing the total size of the study.
B) using the dependent t test rather than independent t test.
C) decreasing the variability of response within groups.
D) driving the means further apart.