Results from a research study suggest that a stop-smoking program is very successful. However, the participants who volunteered for the study were all highly motivated to quit smoking and the...

Results from a research study suggest that a stop-smoking program is very successful. However, the participants who volunteered for the study were all highly motivated to quit smoking and the researcher is concerned that the same results may not be obtained for smokers who are not as motivated. This study has a problem with

a. internal validity.

b. external validity.

c. experimental validity.

d. validity of measurement.