Suppose Sally grew a wild type E. coli culture in rich liquid media that contained all 20 amino acids until the culture was dividing exponentially, with one cell division approximately every 20 minutes. She then added the mutagen 5-bromouracil to the media. After the cells had grown for 20 more minutes, she washed the cells to remove the mutagen and resuspended the washed cells in sterile water. Next, Sally plated the resuspended cells on minimal media supplemented with tryptophan and obtained well-separated colonies, such that each colony arose from a single bacterial cell. She replica-plated these colonies on minimal media and selected a single colony that grew on the media supplemented with tryptophan, but not on minimal media. Sally inoculated 10 test tubes containing fresh minimal media supplemented with tryptophan with cells from this colony and grew the 10 cultures until the cells were dividing exponentially. She then plated 0.1 mL of each culture onto separate plates containing minimal media. No colonies grew on 3 of the 10 plates. Colonies grew on the remaining seven plates, with one plate containing about 100 colonies and the other plates each containing between 1 and 15 colonies. What type of mutation allowed colonies to grow on these seven plates?
a) spontaneous forward mutation
b) spontaneous reverse mutation
c) induced forward mutation
d) induced reverse mutation