Following statements were given regarding factors influencing the efficiency of gene transfer in bacterial transformation:

A. The concentration of the plasmid DNA.

B. The type of bacterial strain used.

C. The presence of antibiotics in the growth medium.

D. The temperature and duration of the heat shock step.

Which one of the following options represents a combination of statements that would NOT lead to variations in gene transfer efficiency in bacterial transformation using the same plasmid and bacterial strain?