A: Mice that are unable either to make macrophages or to make them in sufficient numbers will protect themselves from bacterial infections in some other way.
Nothing is there in the argument that supports this type of conclusion.

B: Mice that show elevated levels of nitrates can easily fight off most types of bacterial infections.
This sort of connection between elevated levels of nitrates and bacterial infections is not reflected in the argument.

C: In mice, macrophages play a role in the production of nitrates or inhibit a process by which nitrates are broken down or otherwise eliminated.
The last line of the argument I,e Unlike …………….bacteria points to this kind of a role played by macrophages

D: When a healthy mouse becomes infected with an invasive organism, the number of macrophages in the mouse body decreases.
This is nowhere indicated in the argument.

E: Injections of nitrates into mice that lack macrophages will not enhance the ability of these animals' immune systems to fight off infection.
This is out of the scope of the argument.

Answer: C