Answer:
A - Mice that are unable to make macrophages or to make them in sufficient numbers will protect themselves from bacterial infections in some other way
Step-by-step explanation:
The statement inferred that macrophages play a role in the immune system of mice and since elevated nitrate levels is an indicator of bacterial infection, the fact that the mice do not show elevated nitrate level means that they must be fighting the bacterial infection in some other way which makes A the only logical conclusion.
B is unacceptable as the mice do not show elevated nitrate levels.
C is far fetched because it talks about the production of nitrates by macrophages while the statement inferred elevated nitrate as a result of bacterial infection.
D is false because it talks about mice able to produce macrophages and E is also wrong because it talks about injection of nitrates into mice which is not related to the statement