In an Immunodiffusion Test to Diagnose Histoplasmosis, a Patient's Serum

In an immunodiffusion test to diagnose histoplasmosis, a patient's serum is placed in a well in an agar plate. In a positive test, a precipitate forms as the serum diffuses from the well and meets material diffusing from a second well. In this test process, what is in the second well?

A) mycelia
B) red blood cells
C) antibodies
D) fungal cells
E) a fungal antigen