SAT Transitions Question

Firefly luciferase (Fluc) is a distinctly evolved enzyme that can oxidize a substrate called D-luciferin to induce bioluminescence and can act as a fatty acyl CoA synthetase (ACS) enzyme, a class of enzyme present in all insects. Fluc is a bifunctional enzyme whose presence indicates an insect is capable of emitting light; in contrast, the presence of ACS alone would be insufficient for determining bioluminescence. Which choice completes the next with the most logical transition?