Which of the following is an example of horizontal gene transfer?

a. The transfer of a gene from one strain of E. coli to a different strain via conjugation

b. The transfer of a gene from one strain of E. coli to a different strain via transduction

c. The transfer of an antibiotic resistance gene from E. coli to Salmonella typhimurium via transformation

d. All of the above are examples of horizontal gene transfer.