Multiple copies of an antibiotic resistance gene was found on the chromosome isolated from bacteria X. What is the most likely explanation for this occurrence?

A、Multiple copies of an antibiotic resistance gene was found on the chromosome isolated from bacteria X. What is the most likely explanation for this occurrence?
B、the bacteria mated with numerous bacteria containing plasmids with the antibiotic resistance gene
C、the bacteria contained the antibiotic resistance gene on a transposon that underwent replicative transposition
D、the bacteria contained multiple copies of the antibiotic resistance gene from recombination