Context: In a group of rats, some individuals have a normal-sized body and others have a dwarf body. In this group, the gene for the body size trait has two alleles. The allele for a dwarf body (b) is recessive to the allele for a normal-sized body (B).
Amos is a rat from this group. Amos has the heterozygous genotype Bb for the body size gene.
Based on this information, what is Amos's phenotype for the body size trait?
A. a normal-sized body
B. a dwarf body
Answer with the option's letter from the given choices directly.

A