Many Students Often Develop Colds and Other Minor Illnesses During

Many students often develop colds and other minor illnesses during final exam week.Why does this happen,based on the research into the physical responses to stress

A)  Stress raises the level of cholesterol in the students' blood. 
B)  Stress lowers the level of cortisol in the students' blood. 
C)  Stress reduces the level of immune system activity. 
D)  Stress raises the level of neuropeptides and amino acids.