The single-case experimental design a. often incorporates several people at once. b. is not concerned with external validity. c. always uses a single case. d. is not concerned with internal validity.

The correct answer to the question is C) .

Explanation:

The single case experiment design which can also be termed as single subject experimental design, is applied when the sample size or case is one .

This type of experiment is used to study the change in an individual's behavior and these changes occurs because of some treatment or intervention that an individual might have received.

answercombination of ability and experience of the partners;