Which of the following is mentioned in the passage as a drawback of the immunological method of detecting peptide hormones? A. It cannot be used to detect the presence of growth regulators in the brain. B. It cannot distinguish between the peptide hormones and substances that are very similar to them. C. It uses antiserums that are unable to cross the blood-brain barrier. D. It involves a purification process that requires extensive training in endocrinology. E. It involves injecting foreign substances directly into the bloodstream.