Patients with Chronic Obstruction Pulmonary Disease (COPD) Volunteered to Be

Patients with chronic obstruction pulmonary disease (COPD) volunteered to be tested for exhaled nitric oxide (eNO) while resting and after a 6 minute walk test. Each subject was measured under each condition, and the order of presentation (resting then walking or the other way around) was randomized. Researchers believe the eNO should be higher while walking than resting for COPD patients. The data are summarized in the table below. Assume each of the distributions of eNO - while walking, while resting, and the differences - is not heavily skewed.
-Which of the following best describes the parameter of interest?

A) The true difference in mean exhaled nitric oxide in COPD patients between those who are walking and those who are resting
B) The true mean difference in exhaled nitric oxide in COPD patients while walking compared to while resting
C) The true proportion of COPD patients whose exhaled nitric oxide is higher while walking than while resting
D) The true correlation between the exhaled nitric oxide of COPD patients while walking and while resting