The Major Difference Between Normal Hemoglobin and That of Sickle $\beta$

The major difference between normal hemoglobin and that of sickle cell anemia is

A) sickle cell hemoglobin is the L isomer and the normal is the D isomer.
B) sickle cell hemoglobin is a much smaller molecule than normal hemoglobin.
C) sickle cell hemoglobin is assembled in a reverse manner.
D) sickle cell hemoglobin has one different amino acid in each $\beta$ -chain.