A 13-year-old patient is brought to her pediatrician with a 2-week history of fatigue, an occasional low-grade fever, and malaise. The pediatrician indicates it is a possible infection but needs to know what type of infection. She orders a hematology laboratory test to determine the relative number and percentage of each type of leukocytes. This test is referred to as a:
A) CBC
B) WBC diff.
C) Hemoglobin determination
D) Hematocrit