Answer:
To use a paired t-test
Step-by-step explanation:
t-tests are essentially tests carried out to compare two different groups to check if there is a major difference in their performances or results which may be related in certain characteristics.
A paired (correlated) t-test is used when there are matched pair of similar units or instances of repeated measurements.
In this case, Mary has to use a paired t-test because the samples (students) have high similarity in characteristics. The students in question has to be from the same class to test her hypothesis (similar characteristics).