Escherichia coli strains X and Y are both resistant to ampicillin. Ampicillin resistance is stable in strain X when it is grown for multiple generations in the absence of the antibiotic. However, strain Y loses ampicillin resistance when it is grown in media without the antibiotic. Which of the following best explains the acquisition of ampicillin susceptibility in strain Y?
(A) Downregulation of the resistance gene
(B) Insertion of a transposon into the resistance gene
(C) Loss of a plasmid carrying the resistance gene
(D) Point mutations in the resistance gene
(E) Recombination with a defective copy of the resistance gene