There Are Two Alleles for Each Gene in a Cell

There are two alleles for each gene in a cell.If each gene encodes for a particular protein,what would make two alleles different from each other?

A) The sequence of bases in the DNA for each allele would be slightly different and would lead to the production of slightly different proteins.
B) The sequence of bases in the DNA would be different for each allele, but they would result in the production of identical proteins.
C) The sequence of bases in the DNA would be identical for each allele, but they would produce very different proteins.
D) The sequence of bases in the DNA would be identical for each allele, but one would produce a protein and the other would not.