In the context of psychological research methods, which of the following refers to a carefully regulated procedure in which the researcher manipulates one or more variables that are believed to influence some other variable?

Phenylketonuria (PKU):

a. Is an inability to make proteins b. Interferes with myelination c. Can be prevented by increasing the amount of phenylalanine in the diet d. Involves excess proteins that build up from an extra chromosome 21

Classical conditioning can be thought of as a type of ____

a. associative learning b. imitation c. instrumental learning d. vicarious learning