What Is the Simplest Definition of a Genetic "Logic Gate

What is the simplest definition of a genetic "logic gate"?

A) a promoter (which interacts with input signals such as regulatory proteins) and a gene, which produces mRNA and protein (output signal)
B) The interaction among proteins and specific DNA sequences modulate gene expression.
C) the combination of genetic elements from different organisms to engineer a novel metabolic pathway or reproduce one in vivo
D) determine whether a gfp gene is turned on or off
E) bind inducer molecules and have an effect on gene expression