Which of the following combinations would be least likely to influence the level of transgene expression in transgenic plants generated using a standard binary vector system?

A. The choice of selectable marker gene
B. The chromatin structure surrounding the transgene integration site
C. The presence of post-transcriptional gene silencing mechanisms
D. The size of the T-DNA region inserted into the plant genome

Which one of the following combinations would NOT lead to significant variations in transgene expression?