What Actually Happens to the Blood Cells of People with Sickle

What actually happens to the blood cells of people with sickle cell anemia that causes the cells to change their shape ("sickle") ?

A) Polymerization of hemoglobin molecules into long fibers.
B) Destruction of the hemoglobin molecules.
C) Loss of hemoglobin through leaky cell membranes.
D) Attachment of phosphate groups to the hemoglobin molecules.
E) Binding of methyl groups to the heme group of the hemoglobin molecule.