Ten sets of identical twins, all wanting to learn French, were divided into two groups, each group containing one of each twin pair. Group 1 was flown to France, where they lived for one month. Group 2 was enrolled in an intensive French course at a local university. At the end of one month, all subjects were given a standard French language exam. Which procedure is appropriate for performing the analysis of the exam scores?
A. One-sample z-test for a mean
B. Two-sample z-test for means
C. One-sample t-test for a mean
D. Two-sample t-test for means
E. One sample z-test for proportion