question: What are the flattened, disk-shaped cells in blood that carry oxygen?
distractor3: white blood cells
distractor1: plasma
distractor2: platlets
correct_answer: red blood cells
support: Red blood cells (RBCs) are flattened, disk-shaped cells that carry oxygen. They are the most common blood cell in the blood. There are about 4 to 6 million RBCs per cubic millimeter of blood. Each RBC has about 200 million molecules of hemoglobin. Hemoglobin is the protein that carries oxygen. Hemoglobin also gives the red blood cells their red color.