Macrophages are cells that play a role in the response of the immune system of mice and other mammals to invasive organisms such as bacteria. Unlike other mice, mice that are genetically incapable of making these particular cells do not show elevated levels of nitrates when infected with bacteria. The statements above, if true, provide the most support for which of the following conclusions? ["Injections of nitrates into mice that lack macrophages will not enhance the ability of these animals' immune systems to fight off infection.", 'In mice, macrophages play a role in the production of nitrates or inhibit a process by which nitrates are broken down or otherwise eliminated.', "When a healthy mouse becomes infected with an invasive organism, the number of macrophages in the mouse's body decreases.", 'Mice that show elevated levels of nitrates can easily fight off most types of bacterial infections.']