Below Are Short Fragments of DNA Sequences from Two Yeast

Below are short fragments of DNA sequences from two yeast strains (A and B) , which have been diverging from each other, as well as a short fragment from their common ancestor (C) .
Yeast A: TGACTGCAAACATCCTAG
Yeast B: TGCCTGGACACATCGAAG
Yeast C: TGCCTGGAAACCTCCTAG

What type of information would best allow you to detect whether back substitutions occurred at any of the 18 nucleotides of the sequence as strain A evolved from the common ancestor?

A) Additional sequences from strain A from regions near the 18 nucleotides
B) Additional sequences from the ancestor from regions near the 18 nucleotides
C) Phenotypic data from strain A and the ancestor
D) The 18 nucleotides from yeast samples taken at various times between the ancestor and contemporary strain A
E) Sequence data from closely related strains