You have three tubes of nitrate broth, inoculated with diffe…

You have three tubes of nitrate broth, inoculated with different organisms. Tube A turns red after the addition of nitrate reagents A and B, tube B turns red after the addition of zinc (it was colorless after reagents A and B), tube C remains colorless after the addition of zinc (it was also colorless after reagents A and B were added. Which tube is negative for the ability of the organisms to break down the substrate nitrate?