When researchers examined the effects of withholding nitrogen from Bradyrhizobium japonicum they ...

When researchers examined the effects of withholding nitrogen from Bradyrhizobium japonicum they used a split root treatment in which half of a soybean plant's root mass received gaseous nitrogen, whereas the other half did not.
 
  Why did the researchers use this split root treatment?
  a. They economized on experimental soybean plants.
  b. They tested whether different parts of the root react differently to the absence of nitrogen.
  c. They tested the range of nitrogen in which soybean plants can thrive.
  d. They tested whether the potential retaliation against cheating symbionts was specific to root nodules that did not fix nitrogen.