Human immunodeficiency virus (HIV) infects CD4+ T-cells and ultimately kills them. Untreated HIV infection results in the acquired immune deficiency syndrome (AIDS). Which of the following immune system defects in AIDS patients is/are directly caused by the loss of CD4+ T cells?
a) Impaired antibody production
b) Decreased activation of cytotoxic T cells
c) Reduced activation of B cells
d) All of the above