Which Is a Characteristic of the Residual Volume of Human

Which is a characteristic of the residual volume of human lungs?

A) It is larger in healthy people than in people with respiratory disease.
B) It equals the functional residual volume plus the expiratory reserve volume.
C) It contributes to the dilution of oxygen in inhaled air.
D) It can be measured directly with a spirometer.
E) There are no indirect methods for determining it.