Manny is using a computer model to investigate the effects of substitution mutations. The model presents Manny with a long base sequence for a gene. Manny then selects one base for a mutation, such as by substituting cytosine, "C", for adenine, "A".

Manny completes many trials of mutations. In most trials, the result of the substitution mutation is either no effect, or a polypeptide in which one amino acid is altered. In one trial, however, the result is a truncated polypeptide. Many of its terminal amino acids are missing.

What is the most likely explanation for the truncated polypeptide.

A. The substituted base caused a frameshift mutation.
B. The mutation introduced a stop codon in the middle of the gene.
C. The mutation introduced a start codon in the middle of the gene.
D. The substituted base altered the genetic code.