Studies of a Soybean Plant (Glycine Max) and the Bacterium

Studies of a soybean plant (Glycine max) and the bacterium Bradyrhizobium japonicum have shown that after the bacteria enter the root, cells of the root cortex begin to divide. What is this a response to?

A) to a flavonoid released by soybean roots
B) to nod gene products produced by soybean roots
C) to nod gene products produced by the bacterium
D) to nitrogen fixation by the bacterium