Which of the following statements is correct?
- APeople who are heterozygous for the sickle-cell allele have a resistance to malaria.
- BSickle-cell anaemia is caused by a change in the amino acid sequence of the haemoglobin gene.
- CSickle-cell anaemia is caused by both genetic and environmental factors interacting.
- DThe sickle-cell allele is rare in human populations in areas where there is malaria.