Many physicians did not realize that bleeding was an ineffective treatment for typhoid fever until researchers made effective use of

A) stress inoculation training.
B) the placebo effect.
C) a control group.
D) exposure therapies.

The use of a control group is essential to identify whether a treatment is effective or not. In this case, the researchers were able to compare the outcomes of patients who received bleeding treatment to those who did not, and this comparison led to the realization that bleeding was ineffective in treating typhoid fever. The other options (stress inoculation training, placebo effect, exposure therapies) are not relevant to the discovery regarding bleeding treatment for typhoid fever.