Which of the following statements is NOT true of vaccine adjuvants?
a. options: Formaldahyde and mercury are used as vaccine adjuvants
b. Antigenic subunits are sometimes used as adjuvants to enhance the response to particular structures on target pathogens
c. Adjuvants are added to vaccines to enhance the innate immune response to the antigens
d. Environmental exposure to the substances used as adjuvants far exceeds the amount used in vaccines
