"Unlike other mice, mice that are genetically incapable of making macrophages do not show elevated levels of nitrates when infected with bacteria." means:
Normal mice: infected with bacteria --> making macrophages --> elevated levels of nitrates
Mice incapable of making macrophages: infected with bacteria --> no making of macrophages -> no elevated levels of nitrates
You can see that macrophages and level of nitrates are positively correlated. Therefore, C can be reasonable inferred. | 
