You Have Taken E

You have taken E. coli strain A which has its own plasmids, and mixed it with E. coli B strain without plasmids. E. coli B cells now have plasmids but, in addition, they also carry some genes from E. coli A strain. What has happened is

A) the E. coli B cells have become mutants of the original culture.
B) vertical transmission of DNA has occurred.
C) an example of transformation.
D) the cells of E. coli A were Hfr cells.