To Test Whether Plant Secondary Compounds Enhance the Immune System

To test whether plant secondary compounds enhance the immune system, 12 men stayed for 3 nights in a hotel where plant oils were wafted around the room. At the end of the experiment, scientists found that the men's immune functions were elevated as compared to their immune functions on a normal workday. To better understand these results, you might compare these results to the immune functions of the men after 3 days

A) camping in the forest.
B) in a hotel room without the oils wafting through the room.
C) sharing a hotel room.
D) of isolation from all other humans.