In sickle cell anemia -

• A
  Mutant hemoglobin molecules undergo polymerization under low oxygen tension,causing the change in the shape of the $RBC$.
• B
  Glutamic acid at the sixth position of the $\beta$-globin chain is replaced by valine.
• C
  Mutant hemoglobin molecules undergo polymerization under high oxygen tension,causing the change in the shape of the $RBC$.
• D
  The $\alpha$-globin chain is altered.