In Order to Test the Effectiveness of a Drug Called

In order to test the effectiveness of a drug called XZR designed to reduce cholesterol levels, the cholesterol levels of 9 heart patients are measured before they are given the drug. The same 9 patients use XZR for two continuous months. After two months of continuous use, the cholesterol levels are measured again. The comparison of cholesterol levels before versus after administering the drug is an example of testing the difference between

A) two means from independent populations.
B) two population variances from independent populations.
C) two population proportions.
D) matched pairs from two dependent populations.