A Sickle Cell Allele Is Potentially Fatal, Yet It's Not

A sickle cell allele is potentially fatal, yet it's not all bad. Why not?

A) Alleles always interact, so not all sickle cell genes are fatal. Some people live with the disease.
B) It isn't fatal if you have one just one sickle cell allele and in that case offers resistance to malaria.
C) It is all bad.
D) It isn't fatal if you have just one sickle cell allele and in that case offers resistance to lots of other diseases.
E) Scientists learn something about curing genetic diseases in general every time someone comes down with sickle cell disease.