Teacher:You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Teacher: Now, understand the problem? Solve this instance: Sickle cell anemia is a condition that affects the shape of a red blood cell. An unaffected individual can be represented with the genotype AA. An affected individual can be represented with the genotype SS. Individuals that are carriers have the genotype AS. Which is the best description of sickle cell anemia?  
 (A) multiple alleles (B) incomplete dominance (C) homozygous recessive (D) heterozygous dominant
Student:
B