7. Sickle cell anemia is a molecular disease of hemoglobin. The altered properties of hemoglobin S result from a single amino acid mutation. The following statement which is right? (A) a Leu instead of a Asp residue (B) a Val instead of a Glu residue (C) a Ile instead of a Asn residue (D) a Leu instead of a Gln residue (E) a Met instead of a Asn residue