A blood test performed on whole blood to determine the percentage of RBCs in the total blood volume is:

A) RBC.
B) WBC.
C) Hct.
D) PTT.

Hct (hematocrit) is a blood test that measures the percentage of RBCs in the total blood volume. This is calculated by centrifuging a blood sample, which causes the RBCs to settle at the bottom of the tube, and then measuring the height of the RBCs as a percentage of the total blood volume. RBC (choice A) would not be correct as it refers to a specific type of blood cell, not a test for determining the percentage of RBCs. WBC (choice B) refers to white blood cells, which are not involved in determining the percentage of RBCs. PTT (choice D) is a test for measuring the clotting ability of blood and is not related to the percentage of RBCs.